Group iii nitride composite substrate and method for manufacturing the same, and method for manufacturing group iii nitride semiconductor device

ABSTRACT

Provided are a group III nitride composite substrate having a low sheet resistance and produced with a high yield, and a method for manufacturing the same, as well as a method for manufacturing a group III nitride semiconductor device using the group III nitride composite substrate. A group III nitride composite substrate includes a group III nitride film and a support substrate formed from a material different in chemical composition from the group III nitride film. The group III nitride film is joined to the support substrate in one of a direct manner and an indirect manner. The group III nitride film has a thickness of 10 μm or more. A sheet resistance of a group III-nitride-film-side main surface is 200 Ω/sq or less.

TECHNICAL FIELD

The present invention relates to a group III nitride composite substrateand a method for manufacturing the same, as well as a method formanufacturing a group III nitride semiconductor device using the groupIII nitride composite substrate.

BACKGROUND ART

Group III nitride semiconductors such as GaN, AlN, Al_(x)Ga_(1-x)N(0<x<1), and the like have superior semiconductor properties and aretherefore suitable for a substrate of a semiconductor device. Such groupIII nitride semiconductors are expensive. Therefore, in order to reducethe cost of manufacturing a semiconductor device, there has beenproposed a substrate of a semiconductor device, specifically asemiconductor substrate in which a film of a group III nitridesemiconductor such as GaN, AlN, or the like is formed on a supportsubstrate such as silicon substrate.

For example, Japanese Patent Laying-Open No. 2006-210660 (PTD 1)discloses a method for manufacturing a semiconductor substrate includingthe steps of: implanting ions into a front surface and its vicinity of afirst nitride semiconductor substrate made from GaN, AlN, or the like;superposing a second substrate on the front surface of the first nitridesemiconductor substrate; heat-treating the two substrates superposed oneach other; and stripping most of the first nitride semiconductorsubstrate from the second substrate along a layer in which the ions areimplanted.

CITATION LIST Patent Document

-   PTD 1: Japanese Patent Laying-Open No. 2006-210660

SUMMARY OF INVENTION Technical Problem

In a semiconductor substrate produced by the method for manufacturing asemiconductor substrate disclosed in Japanese Patent Laying-Open No.2006-210660 (PTD 1), the thickness of the nitride semiconductor formedon the support substrate is a thin thickness on the order of 2 μm, andthus the semiconductor substrate suffers from a problem of a high sheetresistance of the substrate, and a problem of a low yield because thesubstrate is partially of an increased resistance due to damage causedby the ion implantation.

The present invention aims to solve the above problems and provide agroup III nitride composite substrate having a low sheet resistance andproduced with a high yield, and a method for manufacturing the same, aswell as a method for manufacturing a group III nitride semiconductordevice using the group III nitride composite substrate.

Solution to Problem

According to an aspect of the present invention, a group III nitridecomposite substrate includes a group III nitride film and a supportsubstrate formed from a material different in chemical composition fromthe group III nitride film. Here, the group III nitride film is joinedto the support substrate in one of a direct manner and an indirectmanner. The group III nitride film has a thickness of 10 μm or more. Asheet resistance of a group III-nitride-film-side main surface of thegroup III nitride composite substrate is 200 Ω/sq (ohms per square) orless. In this group III nitride composite substrate, the thickness ofthe group III nitride film is 10 μm or more, and therefore, the sheetresistance of the group III-nitride-film-side main surface of the groupIII nitride composite substrate can be reduced to 200 Ω/sq or less, andgroup III nitride semiconductor devices can thus be manufactured with ahigh yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, an area of a joined region joining thegroup III nitride film and the support substrate is 70% or more relativeto an area of the main surface, a non-joined region failing to join thegroup III nitride film and the support substrate includes at least onenon-joined partial region, and the non-joined partial region may be asmall non-joined partial region having a maximum size in radialdirection of less than 20 mm. In this group III nitride compositesubstrate, the area of the joined region is a large area of 70% or morerelative to the area of the main surface and the maximum size in radialdirection of the non-joined partial region forming the non-joined regionis a small size of less than 20 mm. Therefore, group III nitridesemiconductor devices can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, a non-joined region failing to join thegroup III nitride film and the support substrate includes at least onenon-joined partial region, and the non-joined partial region may be aninner non-joined partial region failing to abut on a perimeter of themain surface. In this group III nitride composite substrate, thenon-joined partial region forming the non-joined region does not abut onthe perimeter of the main surface. Therefore, group III nitridesemiconductor devices can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, the group III nitride film has amain-surface through hole, and an area of the main-surface through holemay be 10% or less relative to an area of the main surface. In thisgroup III nitride composite substrate, the area of the main-surfacethrough hole is 10% or less relative to the area of the main surface.Therefore, group III nitride semiconductor devices can be manufacturedwith a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, a joint interface between the group IIInitride film and the support substrate includes an impurity containingmetal, and the concentration of the impurity may be 1×10¹⁰ cm⁻² or more.In this group III nitride composite substrate, the concentration of theimpurity containing metal in the joint interface is 1×10¹⁰ cm⁻² or more.Therefore, group III nitride semiconductor devices with a high jointstrength can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, the group ITT nitride film may have athermal expansion coefficient of more than 0.7 times and less than 1.4times as large as a thermal expansion coefficient of the supportsubstrate. In this group III nitride composite substrate, the thermalexpansion coefficient of the group III nitride film is more than 0.7times and less than 1.4 times as large as the thermal expansioncoefficient of the support substrate. Therefore, in the step of applyingheat during manufacture of a group III nitride semiconductor device,occurrence of warp and/or cracks is prevented, and accordingly group IIInitride semiconductor devices can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, the support substrate may have afracture toughness of 1 MNm⁻²M or more, and the support substrate mayhave a thickness of 50 μm or more. In this group III nitride compositesubstrate, the mechanical strength is high. Therefore, group III nitridesemiconductor devices can be manufactured with a high yield.

In the group III nitride composite substrate according to the aboveaspect of the present invention, the indirect manner may be a manner ofinterposing a joint film between the group III nitride film and thesupport substrate. In this group III nitride composite substrate, thegroup III nitride film and the support substrate are joined to eachother with the joint film interposed therebetween. Therefore, group IIInitride semiconductor devices having a high joint strength can bemanufactured with a high yield.

A method for manufacturing a group III nitride composite substrateaccording to another aspect of the present invention is a method formanufacturing a group III nitride composite substrate of the aboveaspect, and includes the steps of: bonding the group III nitride filmand the support substrate to each other in one of a direct manner and anindirect manner; and reducing the thickness of at least one of the groupIII nitride film and the support substrate bonded to each other. Thismethod for manufacturing a group III nitride composite substrateincludes these steps so that group III nitride composite substrates witha low sheet resistance can be manufactured with a high yield.

A method for manufacturing a group III nitride semiconductor deviceaccording to still another aspect of the present invention is a methodfor manufacturing a group III nitride semiconductor device using a groupIII nitride composite substrate of the above aspect, and includes thesteps of: preparing the group III nitride composite substrate; andgrowing at least one group III nitride layer on the groupIII-nitride-film-side main surface of the group III nitride compositesubstrate. This method for manufacturing a group III nitridesemiconductor device includes these steps so that group III nitridesemiconductor devices can be manufactured with a high yield.

Advantageous Effects of Invention

In accordance with the present invention, a group III nitride compositesubstrate having a low sheet resistance and produced with a high yield,and a method for manufacturing the same, as well as a method formanufacturing a group III nitride semiconductor device using the groupIII nitride composite substrate can be provided.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is a schematic cross-sectional view showing an example of thegroup III nitride composite substrate according to the presentinvention.

FIG. 2 is a schematic cross-sectional view showing another example ofthe group III nitride composite substrate according to the presentinvention.

FIG. 3 is a schematic plan view showing a joined region and non-joinedregions in the group III nitride composite substrate according to thepresent invention.

FIG. 4 is a schematic cross-sectional view showing an example of themethod for manufacturing a group III nitride composite substrateaccording to the present invention.

FIG. 5 is a schematic cross-sectional view showing an example of thegroup III nitride semiconductor device produced by the method formanufacturing a group III nitride semiconductor device according to thepresent invention.

FIG. 6 is a schematic cross-sectional view showing an example of thegroup III nitride composite substrate according to a referenceinvention.

FIG. 7 is a schematic plan view showing measurement points at whichmeasurements of physical properties are taken in the group III nitridecomposite substrate.

FIG. 8 is a schematic cross-sectional view showing an example of alaminated group III nitride composite substrate according to thereference invention.

FIG. 9 is a schematic cross-sectional view showing an example of thegroup III nitride semiconductor device according to the referenceinvention.

FIG. 10 is a schematic cross-sectional view showing another example ofthe group III nitride semiconductor device according to the referenceinvention.

FIG. 11 is a schematic cross-sectional view showing still anotherexample of the group III nitride semiconductor device according to thereference invention.

FIG. 12 is a schematic cross-sectional view showing an example of amethod for manufacturing a group III nitride composite substrateaccording to the reference invention.

FIG. 13 is a schematic cross-sectional view showing another example ofthe method for manufacturing a group III nitride composite substrateaccording to the reference invention.

FIG. 14 is a schematic cross-sectional view showing still anotherexample of the method for manufacturing a group III nitride compositesubstrate according to the reference invention.

FIG. 15 is a schematic cross-sectional view showing an example of themethod for manufacturing a group III nitride semiconductor deviceaccording to the reference invention.

FIG. 16 is a schematic cross-sectional view showing another example ofthe method for manufacturing a group III nitride semiconductor deviceaccording to the reference invention.

FIG. 17 is a schematic cross-sectional view showing an example of themethod for manufacturing a group III nitride composite substrate throughthe use of ion implantation.

DESCRIPTION OF EMBODIMENTS First Embodiment: Group III Nitride CompositeSubstrate

Referring to FIGS. 1 and 2, a group III nitride composite substrate 1which is an embodiment of the present invention includes a group IIInitride film 13 and a support substrate 11 formed from a material whichis different in chemical composition from group III nitride film 13.Here, group III nitride film 13 is joined to support substrate 11 in oneof a direct manner and an indirect manner. Group III nitride film 13 hasa thickness of 10 μm or more. A sheet resistance of a groupIII-nitride-film 13-side main surface 13 m, which is a main surface onthe group III-nitride-film side, is 200 Ω/sq or less.

In group III nitride composite substrate 1 of the present embodiment,group III nitride film 13 has a thickness of 10 μm or more, andtherefore, the sheet resistance of group III-nitride-film 13-side mainsurface 13 m can be reduced to 200 Ω/sq or less. Thus, group III nitridesemiconductor devices can be manufactured with a high yield. Detailswill be described in the following.

Group III nitride composite substrate 1 of the present embodimentincludes group III nitride film 13, and support substrate 11 formed froma material different in chemical composition from group III nitride film13.

<Group III Nitride Film>

Group III nitride film 13 is a substrate formed from a semiconductorwhich is a compound of at least one group III element and nitrogen, andthe substrate may for example be GaN substrate, AlN substrate,Al_(x)Ga_(1-x)N substrate (0<x<1), or the like. The method formanufacturing this group III nitride film 13 is not particularlylimited. This method may be a gas phase method such as HVPE (HydrideVapor Phase Epitaxy), sublimation method, or the like, may be a liquidphase method such as high nitrogen pressure solution method, fluxmethod, or the like. In order to grow a group III nitride layer of highcrystal quality on group III nitride film 13, group III nitride film 13is preferably crystal, and more preferably single crystal.

In order to reduce the sheet resistance of group III nitride film 13,group III nitride film 13 may be doped with a conductivity improverimpurity. This conductivity improver impurity is not particularlylimited. O (oxygen) atom, Si (silicon) atom, and the like are suitablesince they are highly effective in improving the conductivity.

<Support Substrate>

Support substrate 11 is a substrate supporting group III nitride film13, and is not particularly limited as long as it is a substrate formedfrom a material which is different in chemical composition from groupIII nitride film 13. The support substrate may be an oxide substrate,and examples of the oxide substrate are: sapphire substrate and otherAl₂O₃ substrates; mullite substrate and other Al₂O₃—SiO₂-basedsubstrates; spinel substrate and other Al₂O₃—MgO-based substrates;Al₂O₃—SiO₂—YSZ (Yttria Stabilized Zirconia)-based substrate, and thelike. The support substrate may also be a metal substrate such as Mosubstrate, W substrate, Cu—W substrate, or the like. It may also be Sisubstrate, SiC substrate, graphite substrate, or the like. In addition,in the case where group III nitride film 13 is a GaN film, the supportsubstrate may be a substrate formed from AlN for example which is agroup III nitride different in chemical composition from GaN. Supportsubstrate 11 may be either crystal or amorphous. In the case wheresupport substrate 11 is crystal, it may be either single crystal orpolycrystal.

<Manner of Joining Group III Nitride Film and Support Substrate>

In group III nitride composite substrate 1 of the present embodiment,group III nitride film 13 is joined to support substrate 11 in one of adirect manner and an indirect manner.

Referring to FIG. 1, the joint in a direct manner means that group IIInitride film 13 and support substrate 11 are directly joined to eachother without a separate element interposed therebetween. In the case ofthis direct joint manner, a joint interface 100 is an interface formedby a main surface 13 n of group III nitride film 13 and a main surface11 m of support substrate 11 that are joined to each other.

Referring to FIG. 2, the joint in an indirect manner means that groupIII nitride film 13 and support substrate 11 are indirectly joined toeach other with a separate element such as a joint film 12 for exampleinterposed therebetween. In the case of this indirect joint manner, thejoint interface varies depending on how the substrate and the film arebonded to each other as described below.

A first bonding method is as follows. As shown in FIGS. 2 and 4, a jointfilm 12 a is formed on main surface 11 m of support substrate 11 and ajoint film 12 b is formed on main surface 13 n of group III nitride film13. After this, a main surface 12 am of joint film 12 a and a mainsurface 12 bn of joint film 12 b are bonded to each other to therebyform joint film 12 into which joint film 12 a and joint film 12 b areintegrated. In this case, joint interface 100 is a surface formed bymain surface 12 am of joint film 12 a and main surface 12 bn of jointfilm 12 b that are joined to each other in joint film 12.

A second bonding method is as follows. Joint film 12 is formed on mainsurface 11 m of support substrate 11 and thereafter the main surface ofjoint film 12 and main surface 13 n of group III nitride film 13 arebonded to each other. In this case, joint interface 100 is a surfaceformed by the main surface of joint film 12 and main surface 13 n ofgroup III nitride film 13 that are joined to each other.

A third bonding method is as follows. Joint film 12 is formed on mainsurface 13 n of group III nitride film 13, and thereafter the mainsurface of joint film 12 and main surface 11 m of support substrate 11are bonded to each other. In this case, joint interface 100 is a surfaceformed by the main surface of joint film 12 and main surface 11 m ofsupport substrate 11 that are joined to each other.

<Joint Film>

Joint film 12 which may be included between group III nitride film 13and support substrate 11 in group III nitride composite substrate 1 ofthe present embodiment is not particularly limited as long as the jointfilm serves to increase the strength with which group III nitride film13 and support substrate 11 are joined. The joint film may be SiO₂ film,Si₃N₄ film, AlN film, Al₂O₃ film, TiO₂ film, TiN film, Ga₂O₃ film, Wfilm, Mo film, Au—Sn film, or the like.

The thickness of joint film 12 is not particularly limited. For the sakeof increasing the strength with which group III nitride film 13 andsupport substrate 11 are joined to each other, the thickness ispreferably 0.05 μm or more, and more preferably 0.1 μm or more. For thesake of improving the in-plane uniformity and the flatness of joint film12, the thickness is preferably 5 μm or less, and more preferably 2 μmor less.

<Thickness of Group III Nitride Film>

Regarding group III nitride composite substrate 1 in the presentembodiment, for the sake of reducing the sheet resistance of groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1, the thickness of group III nitride film 13 needsto be 10 μm or more, and is preferably 50 μm or more, and morepreferably 100 μm or more. For the sake of reducing the cost of groupIII nitride composite substrate 1, the thickness of group III nitridefilm 13 is preferably 500 μm or less, and more preferably 250 μm orless.

<Sheet Resistance of Group III-Nitride-Film-Side Main Surface>

Regarding group III nitride composite substrate 1 in the presentembodiment, for the sake of improving device properties of a group IIInitride semiconductor device (such as luminous efficiency of alight-emitting device, for example) that are achieved by reducing thesheet resistance of group III nitride composite substrate 1, the sheetresistance of group III-nitride-film 13-side main surface 13 m of groupIII nitride composite substrate 1 needs to be 200 Ω/sq (ohms per square)or less, and is preferably 50 Ω/sq or less, and more preferably 10 D/sqor less.

<Joined Region Joining Group III Nitride Film and Support Substrate andNon-Joined Region Failing to Join Group III Nitride Film and SupportSubstrate>

Referring to FIGS. 1 to 3, in group III nitride composite substrate 1 ofthe present embodiment, group III nitride film 13 is bonded to supportsubstrate 11 along the above-described joint interface 100. Like a groupIII nitride composite substrate produced by the ion implantation method,group III nitride composite substrate 1 in the present embodiment alsohas surface roughness of respective main surfaces of support substrate11, group III nitride film 13, and joint films 12, 12 a, 12 b, amain-surface through hole 13 h of group III nitride film 13,non-uniformity of bonding, and the like. As a result, in group IIInitride composite substrate 1 of the present embodiment, joint interface100 includes a joined region 100 b where group III nitride film 13 andsupport substrate 11 are joined to each other in either the directmanner or the indirect manner, and a non-joined region 100 n where groupIII nitride film 13 and support substrate 11 are not joined to eachother in either the direct manner or the indirect manner.

The presence, respective positions, respective sizes, and respectiveareas of joined region 100 b and non-joined region 100 n in jointinterface 100 of group III nitride composite substrate 1 in the presentembodiment can be measured by means of an ultrasonic microscope, adefect evaluation apparatus, or the like.

Non-joined region 100 n in joint interface 100 of group III nitridecomposite substrate 1 in the present embodiment includes at least onenon-joined partial region 111 n, 112 n, 121 n, 122 n. Although the shapeof non-joined partial regions 111 n, 112 n, 121 n, 122 n is indefinite,the shape is substantially circular or elliptical. Therefore, the sizeof the non-joined region is evaluated on the basis of the maximum sizein radial direction (maximum diameter). Non-joined partial regions 111n, 112 n, 121 n, 122 n can be classified by size into small non-joinedpartial regions 111 n, 112 n having a maximum size in radial directionof less than 20 mm, and large non-joined partial regions 121 n, 122 nhaving a maximum radial size of 20 mm or more. Non-joined partialregions 111 n, 112 n, 121 n, 122 n can also be classified, by theposition where the region is located, into inner non-joined partialregions 111 n, 121 n which do not abut on a perimeter 1 r of a mainsurface 1 m, and outer non-joined partial regions 112 n, 122 n whichabut on perimeter 1 r of main surface 1 m.

Regarding group III nitride composite substrate 1 in the presentembodiment, in order to manufacture group III nitride semiconductordevices with a high yield, the area of joined region 100 b joining groupIII nitride film 13 and support substrate 11 relative to the area ofmain surface 1 m is preferably 70% or more, and more preferably 85% ormore. If the area of joined region 100 b joining group III nitride film13 and support substrate 11 relative to the area of main surface 1 m isless than 70%, group III nitride film 13 and support substrate 11 aremore likely to be separated from each other during a process ofmanufacturing a group III nitride semiconductor device, which makes itdifficult to increase the yield of group III nitride semiconductordevices.

Regarding group III nitride composite substrate 1 in the presentembodiment, in order to manufacture group III nitride semiconductordevices with a high yield, the non-joined partial region is preferablysmall non-joined partial region 111 n, 112 n having a maximum size inradial direction of less than 20 mm. Namely, in FIG. 3, any non-joinedpartial region is preferably small non-joined partial region 111 n, 112n.

Moreover, regarding group III nitride composite substrate 1 in thepresent embodiment, in order to manufacture group ITT nitridesemiconductor devices with a high yield, the non-joined partial regionis preferably inner non-joined partial region 111 n, 121 n which doesnot abut on perimeter 1 r of main surface 1 m. Namely, in FIG. 3, anynon-joined partial region is preferably inner non-joined partial region111 n, 121 n. In the case of group III nitride composite substrate 1including outer non-joined partial region 112 n, 122 n, contaminants mayenter joint interface 100 during a process of fabricating group IIInitride composite substrate 1 and a process of manufacturing a group IIInitride semiconductor device using group III nitride composite substrate1, and the contaminants are difficult to remove by cleaning. In thiscase, the yield of group III nitride semiconductor devices is difficultto increase. In addition, group III nitride composite substrate 1including outer non-joined partial region 112 n, 122 n is broken throughtreatment during the step of reducing the thickness of at least one ofthe bonded group III nitride film 13 and support substrate 11. In thiscase as well, the yield of group III nitride semiconductor devices isdifficult to increase.

Regarding group III nitride composite substrate 1 in the presentembodiment, in order to manufacture group III nitride semiconductordevices with a high yield, the non-joined partial region is morepreferably small and inner non-joined partial region 111 n. Namely, inFIG. 3, any non-joined partial region is preferably small and innernon-joined partial region 111 n.

<Main-Surface Through Hole of Group III Nitride Film>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to manufacture group IIInitride semiconductor devices with a high yield, the area ofmain-surface through hole 13 h of group III nitride film 13 relative tothe area of main surface 1 m, 13 m is preferably 10% or less, and morepreferably 5% or less. Here, main-surface through hole 13 h is a holeextending through group III nitride film 13 between its main surfacewhich is directly or indirectly joined to support substrate 11 and theother surface (specifically another main surface and side surface).

The presence of main-surface through hole 13 h of group III nitride film13 in group III nitride composite substrate 1 causes non-joined region100 n to be formed on main-surface through hole 13 h and its nearbyregion. When a group III nitride semiconductor device is cleaned in theprocess of manufacturing the group III nitride semiconductor device, acleaning agent may enter the joint interface through main-surfacethrough hole 13 h to thereby cause reduction of the joint strength ofgroup III nitride composite substrate 1. If the area of main-surfacethrough hole 13 h relative to the area of main surface 1 m is 10% ormore, the area of non-joined region 100 n is accordingly larger,resulting in the difficulty in increasing the yield of group III nitridesemiconductor devices.

<Impurity Containing Metal in Joint Interface>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to manufacture group IIInitride semiconductor devices with a high yield by increasing the jointstrength, the concentration of an impurity containing metal that isincluded in joint interface 100 between group III nitride film 13 andsupport substrate 11 is preferably 1×10¹⁰ cm⁻² or more, and morepreferably 1.5×10¹⁰ cm⁻² or more. The impurity containing metal is notparticularly limited. In order to increase the joint strength, theimpurity is preferably an oxide of a base metal such as Fe (iron), Ni(nickel), or the like which has a higher ionization tendency and moreeasily oxidizes than H (hydrogen), in the case where group III nitridecomposite substrate 1 includes, as joint film 12, an oxide film such asSiO₂ film.

<Thermal Expansion Coefficient of Group III Nitride Film and ThermalExpansion Coefficient of Support Substrate>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to suppress occurrenceof warp and/or cracks during manufacture of a group III nitridesemiconductor device and thereby manufacture group III nitridesemiconductor devices with a high yield, group III nitride film 13 has athermal expansion coefficient of preferably more than 0.7 times and lessthan 1.4 times, and more preferably 0.75 times or more and 1.25 times orless as large as a thermal expansion coefficient of support substrate11.

<Fracture Toughness and Thickness of Support Substrate>

Referring to FIGS. 1 to 3, regarding group III nitride compositesubstrate 1 in the present embodiment, in order to suppress occurrenceof warp and/or cracks during manufacture of a group III nitridesemiconductor device, the support substrate has a fracture toughness ofpreferably 1 MNm^(−2/3) or more, and more preferably 1.5 MNm^(−2/3) ormore. In addition, the support substrate has a thickness of preferably50 μm or more, and more preferably 100 μm or more.

Second Embodiment: Method for Manufacturing Group III Nitride CompositeSubstrate

Referring to FIGS. 1, 2, and 4, a method for manufacturing group IIInitride composite substrate 1 which is another embodiment of the presentinvention is a method for manufacturing group III nitride compositesubstrate 1 in the first embodiment, and includes the steps of: bondinggroup III nitride film 13 and support substrate 11 to each other in oneof a direct manner and an indirect manner (FIG. 4 (A)); and reducing thethickness of at least one of group III nitride film 13 and supportsubstrate 11 (FIG. 4 (B)). The method for manufacturing group IIInitride composite substrate 1 in the present embodiment can include thesteps above to thereby manufacture group III nitride compositesubstrates of a low sheet resistance with a high yield.

<Step of Bonding Group III Nitride Film and Support Substrate>

Referring to FIGS. 1, 2, and 4, the method for manufacturing group IIInitride composite substrate 1 in the present embodiment includes thestep of firstly bonding group III nitride film 13 and support substrate11 to each other in one of a direct manner and an indirect manner. Here,bonding group III nitride film 13 and support substrate 11 to each otherin a direct manner means that group III nitride film 13 and supportsubstrate 11 are directly bonded to each other without a separateelement interposed therebetween. Bonding group III nitride film 13 andsupport substrate 11 to each other in an indirect manner means thatgroup III nitride film 13 and support substrate 11 are indirectly bondedto each other with a separate element such as joint film 12 interposedtherebetween.

The method for bonding group III nitride film 13 and support substrate11 to each other is not particularly limited regardless of whether theyare bonded in the direct manner or the indirect manner. Suitable methodsinclude: a direct bonding method according to which respective surfacesto be bonded to each other are cleaned, bonded together as they are, andthereafter heated to approximately 600° C. to 1200° C. so that they arejoined together; a surface activated bonding method according to whichthe surfaces to be bonded to each other are cleaned, subjected to anactivation treatment by means of plasma or ions, and thereafter joinedtogether in a low-temperature atmosphere of room temperature (25° C. forexample) to 400° C.; a high-pressure bonding method according to whichthe surfaces to be bonded to each other are cleaned with a chemicalsolution and pure water and thereafter subjected to a high pressure onthe order of 0.1 MPa to 10 MPa so that they are joined together; a highvacuum bonding method according to which the surfaces to be bonded toeach other are cleaned with a chemical solution and pure water, andthereafter joined together in a high vacuum atmosphere on the order of10⁻⁶ Pa to 10⁻³ Pa, and the like. Any of the above-referenced bondingmethods can increase the temperature to approximately 600° C. to 1200°C. after they are joined together to thereby further increase the jointstrength. In particular, the surface activated bonding method, the highpressure bonding method, and the high vacuum bonding method exhibit ahigher effect of increasing the joint strength through the heating toapproximately 600° C. to 1200° C. after they are joined together.

While group III nitride film 13 and support substrate 11 may be bondedto each other in either the direct manner or the indirect manner asdescribed above, in order to increase the joint strength, group IIInitride film 13 and support substrate 11 are preferably bonded to eachother in an indirect manner with joint film 12 interposed therebetweenas shown in FIG. 4 (A). Details will be described below.

Referring to FIG. 4 (A), the step of bonding group III nitride film 13and support substrate 11 to each other in an indirect manner with jointfilm 12 interposed therebetween includes: the sub step of forming jointfilm 12 a on main surface 11 m of support substrate 11 (FIG. 4 (A1));the sub step of forming joint film 12 b on main surface 13 n of groupIII nitride film 13 (FIG. 4 (A2)); and the sub step of bonding jointfilm 12 a formed on main surface 11 m of support substrate 11 and jointfilm 12 b formed on main surface 13 n of group III nitride film 13 toeach other (FIG. 4 (A3)). These sub steps are performed to join andthereby integrate joint film 12 a and joint film 12 b bonded to eachother into joint film 12, and support substrate 11 and group III nitridefilm 13 are joined to each other with joint film 12 interposedtherebetween.

As to the method for manufacturing group III nitride composite substrate1 in the present embodiment as well, due to the presence of surfaceroughness of respective main surfaces of support substrate 11, group IIInitride film 13, and joint films 12, 12 a, 12 b, main-surface throughhole 13 h of group III nitride film 13, non-uniformity of bonding, andthe like, there are formed, in joint interface 100, joined region 100 bwhere group III nitride film 13 and support substrate 11 are joined toeach other in either the direct manner or the indirect manner, andnon-joined region 100 n where group III nitride film 13 and supportsubstrate 11 are not joined to each other in either the direct manner orthe indirect manner. The description of joined region 100 b andnon-joined region 100 n is given above.

The method for forming joint films 12 a, 12 b is not particularlylimited. In order to reduce the cost of forming the joint film, it ispreferable to perform sputtering, vapor deposition, CVD (Chemical VaporDeposition), or the like. The method for bonding support substrate 11and group III nitride film 13 to each other by bonding joint film 12 aand joint film 12 b to each other is not particularly limited, andpreferred methods are direct bonding method, surface activated bondingmethod, high-pressure bonding method, high vacuum bonding method, andthe like as described above.

<Step of Reducing Thickness of at Least One of Group III Nitride Filmand Support Substrate>

Referring to FIG. 4 (B), the method for manufacturing group III nitridecomposite substrate 1 in the present embodiment includes the step ofsubsequently reducing the thickness of at least one of group III nitridefilm 13 and support substrate 11 bonded to each other. Here, the methodfor reducing the thickness of at least one of group III nitride film 13and support substrate 11 is not particularly limited, and may forexample be the method according to which the substrate whose thicknessis to be reduced is cut in parallel with the main surface, the methodaccording to which the main surface of the substrate whose thickness isto be reduced is ground and/or polished, the method according to whichthe main surface of the substrate whose thickness is to be reduced isetched, the method by means of a laser, or the like. The method by meansof a laser is a method according to which a laser beam is applied sothat the focus of the laser beam is located at a predetermined depthfrom the main surface of the substrate. In the case of the method bymeans of a laser, the chemical composition of the region of the positionat a predetermined depth from the main surface of the substrate wherethe laser beam is focused is changed, and accordingly the substrate canbe divided at this region. Since the method by means of a laser changesthe chemical composition of a region of a position at a predetermineddepth from the main surface of the substrate and does not change thechemical composition of the region other than the above-referencedregion, femtosecond laser, picosecond laser, or the like is preferablyused.

Third Embodiment: Method for Manufacturing Group III NitrideSemiconductor Device

Referring to FIG. 5, a method for manufacturing a group III nitridesemiconductor device 4 which is still another embodiment of the presentinvention includes the steps of: preparing group III nitride compositesubstrate 1; and growing at least one group III nitride layer 20 ongroup III-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1. The method for manufacturing group III nitridesemiconductor device 4 in the present embodiment can include the abovesteps to thereby manufacture group III nitride semiconductor deviceswith a high yield.

<Step of Preparing Group III Nitride Composite Substrate>

The method for manufacturing group III nitride semiconductor device 4 inthe present embodiment includes the step of firstly preparing group IIInitride composite substrate 1. This step of preparing group III nitridecomposite substrate 1 is similar to the steps in the method formanufacturing group III nitride composite substrate 1 in the secondembodiment.

<Step of Growing Group III Nitride Layer>

The method for manufacturing group III nitride semiconductor device 4 inthe present embodiment includes the step of subsequently growing atleast one group III nitride layer 20 on group III-nitride-film 13-sidemain surface 13 m of group III nitride composite substrate 1.

The method for growing group III nitride layer 20 is not particularlylimited. In order to grow group III nitride layer 20 of high crystalquality, preferred methods include vapor phase methods such as MOCVD(Metal Organic Chemical Vapor Deposition), MBE (Molecular Beam Epitaxy),HVPE (Hydride Vapor Phase Epitaxy), sublimation method, and the like,and liquid phase methods such as high nitrogen pressure solution method,flux method, and the like.

The structure of group III nitride layer 20 to be grown varies dependingon the type and the function of the group III nitride semiconductordevice. In the case where group III nitride semiconductor device 4 is alight-emitting device, group III nitride layer 20 may be formed on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1 by growing a first-conductivity-type GaN layer201, a first-conductivity-type Al_(x)Ga_(1-x)N layer 202 (here, s meets0<s<1), a light-emitting layer 203, a second-conductivity-typeAl_(t)Ga_(1-t)N layer 204 (here, t meets 0<t<1), and asecond-conductivity-type GaN layer 205, in this order.

The method for manufacturing group III nitride semiconductor device 4 inthe present embodiment may include the step of subsequently formingelectrodes (a first electrode and a second electrode). In group IIInitride layer 20, second-conductivity-type GaN layer 204, light-emittinglayer 203, first-conductivity-type Al_(s)Ga_(1-s)N layer 202, andfirst-conductivity-type GaN layer 201 may each be partially removed bymesa etching to expose a part of first-conductivity-type GaN layer 201.On the exposed main surface of first-conductivity-type GaN layer 201,first electrode 31 may be formed. On the exposed main surface of thesecond-conductivity-type GaN layer, second electrode 32 may be formed.The method for forming first electrode 31 and second electrode 32 is notparticularly limited, and may be CVD (Chemical Vapor Deposition),sputtering, vapor deposition, or the like.

Some reference inventions for the sake of reference for the preventinvention, namely Reference Invention I, Reference Invention II, andReference Invention III will be described in the following.

<<Reference Invention I>>

Reference Invention I relates to a low-cost and large-diameter group IIInitride composite substrate including a group III nitride film having alarge thickness, a small thickness variation, and a high crystalquality, a method for manufacturing the group III nitride compositesubstrate, a laminated group III nitride composite substrate, and agroup III nitride semiconductor device and a method for manufacturingthe same.

[Background Art Regarding Reference Invention I]

Group III nitrides such as GaN have superior semiconductor propertiesand are therefore used as materials suitable for semiconductor devices.

For example, Japanese Patent Laying-Open No. 2009-126722 discloses afree-standing group III nitride substrate to be used as a substrate fora semiconductor device. The free-standing group III nitride substratehas a diameter of 25 mm or more and 160 mm or less and a thickness of100 μm or more and 1000 μm or less. It discloses, as a specific examplethereof, a free-standing GaN substrate having a diameter of 100 mm and athickness of 400 μm.

Japanese Patent Laying-Open No. 2008-010766 discloses aGaN-thin-film-bonded substrate to be used as a substrate formanufacturing a semiconductor device. The GaN-thin-film-bonded substrateincludes a heterogeneous substrate whose chemical composition isdifferent from that of GaN, and a GaN thin film having a thickness of0.1 μl or more and 100 μm or less and bonded to the heterogeneoussubstrate. It discloses, as a specific example thereof, aGaN-thin-film-bonded substrate having a diameter of 50.8 mm andincluding a sapphire substrate and a GaN thin film having a thickness of0.1 μm or 100 μm and bonded to the sapphire substrate.

Japanese Patent Laying-Open No. 2010-182936 discloses a compositesubstrate to be used as a substrate for a semiconductor device. Thecomposite substrate includes a support substrate, a nitridesemiconductor layer, and a joint layer disposed between the supportsubstrate and the nitride semiconductor layer. It discloses, as aspecific example thereof, a composite substrate having a diameter of50.8 mm and including a sapphire substrate, a GaN layer, and a jointlayer formed by press fitting between the substrate and the GaN layer,in which the GaN layer has a thickness of 5 μm to 220 μm.

Problems to be Solved by Reference Invention I

The free-standing group III nitride substrate disclosed in JapanesePatent Laying-Open No. 2009-126722 involves problems that the substrateis manufactured at high cost and therefore very expensive, and that thesubstrate is likely to crack, resulting in difficulty in increasing thediameter of the substrate and decreasing the thickness thereof.

Regarding the GaN-thin-film-bonded substrate having the GaN thin film of0.1 μm in thickness as disclosed in Japanese Patent Laying-Open No.2008-010766, ion implantation is performed to form the GaN thin film,which, however, involves a problem that the ion implantation degradesthe crystal quality of the GaN thin film. In order to enhance thecharacteristics of the semiconductor device to be formed, the thicknessof the GaN thin film is preferably 10 μm or more. An increase inthickness of the GaN thin film, however, involves a problem that thevariation in depth to which ions are implanted from the main surfaceincreases, which accordingly increases the variation in thickness of theGaN thin film in the resultant GaN-thin-film-bonded substrate.

Regarding the GaN-thin-film-bonded substrate having the GaN thin film of100 μm in thickness as disclosed in Japanese Patent Laying-Open No.2008-010766 as well as the composite substrate having the GaN layer of 5μm to 220 μm in thickness as disclosed in Japanese Patent Laying-OpenNo. 2010-182936, both these substrates have a diameter on the order of50.8 mm. Increase of the diameter, however, involves a problem that thevariation, within the main surface, of the thickness of the GaN thinfilm or the GaN layer increases.

In the case where a heterogeneous substrate such as sapphire substratewhose chemical composition and thermal expansion coefficient aredifferent from those of a group III nitride substrate is prepared and athick group III nitride film is grown on the heterogeneous substrate,there arises problems that large warp occurs and cracks are made.

Reference Invention I has been made to solve the problems above and anobject of the invention is to provide a low-cost and large-diametergroup III nitride composite substrate including a group III nitride filmhaving a large thickness, a small thickness variation, and a highcrystal quality, a method for manufacturing the group ITI nitridecomposite substrate, a laminated group III nitride composite substrate,and a group III nitride semiconductor device and a method formanufacturing the same.

Solution to the Problems

According to an aspect, Reference Invention I provides a group IIInitride composite substrate with a diameter of 75 mm or more including asupport substrate and a group III nitride film having a thickness of 10μm or more and 250 μm or less that are bonded to each other, a ratios_(t)/m_(t) of a standard deviation s_(t) of the thickness of the groupIII nitride film, to a mean value nm of the thickness thereof being0.001 or more and 0.2 or less, and a ratio s_(o)/m_(o) of a standarddeviation s_(o) of an absolute value of an off angle between a mainsurface of the group III nitride film and a plane of a predeterminedplane orientation, to a mean value m_(o) of the absolute value of theoff angle thereof being 0.005 or more and 0.6 or less.

Regarding the group III nitride composite substrate according the aboveaspect of Reference Invention I, the group III nitride compositesubstrate may have a warp of 50 μm or less on the groupIII-nitride-film-side main surface, and the group III nitride compositesubstrate may have a total thickness variation (TTV hereinafter) of 30μm or less. Further, a ratio α_(III-N)/α_(S) of a thermal expansioncoefficient α_(III-N) of the group III nitride film to a thermalexpansion coefficient α_(s) of the support substrate may be 0.75 or moreand 1.25 or less, and a ratio t_(III-N)/t_(S) of a thickness t_(III-N)of the group III nitride film to a thickness t_(S) of the supportsubstrate may be 0.02 or more and 1 or less. Further, impurity metalatoms of the main surface of the group III nitride film may be 3×10¹²atoms/cm² or less. Further, the main surface of the group III nitridefilm may have a root mean square roughness (RMS hereinafter) of 3 nm orless. Further, a main surface of the support substrate may have an RMSof 12 nm or less. Further, the group III nitride composite substrate mayhave a diameter of 100 mm or more, or may have a diameter of 125 mm ormore and 300 mm or less. Further, the main surface of the group IIInitride film may have a mean value m_(III-N) of the RMS of 0.1 nm ormore and 2 nm or less, and a standard deviation s_(III-N) of the RMS of0.4 nm or less, and a main surface of the support substrate may have amean value m_(S) of the RMS of 0.3 nm or more and 10 nm or less, and astandard deviation s_(S) of the RMS of 3 nm or less.

According to another aspect, Reference Invention I provides a laminatedgroup III nitride composite substrate including the group III nitridecomposite substrate according to the above aspect, and at least onegroup III nitride layer disposed on the group III nitride film of thegroup III nitride composite substrate.

According to still another aspect, Reference Invention I provides agroup III nitride semiconductor device including the group III nitridefilm in the group III nitride composite substrate according to the aboveaspect, and at least one group III nitride layer disposed on the groupIII nitride film.

According to a further aspect, Reference Invention I provides a methodfor manufacturing a group III nitride composite substrate according tothe above aspect, including the steps of: forming a joined substratewith a diameter of 75 mm or more by bonding a support substrate and agroup III nitride film donor substrate to each other; and forming thegroup III nitride composite substrate by cutting the group III nitridefilm donor substrate in the joined substrate along a plane locatedinwardly at a predetermined distance from a bonded main surface of thegroup III nitride film donor substrate.

According to a still further aspect, Reference Invention I provides amethod for manufacturing a group III nitride composite substrateaccording to the above aspect, including the steps of: forming a joinedsubstrate with a diameter of 75 mm or more by bonding a supportsubstrate and a group III nitride film donor substrate to each other;and forming the group III nitride composite substrate by performing atleast one of grinding, polishing, and etching on a main surface of thegroup III nitride film donor substrate in the joined substrate, the mainsurface being opposite to a bonded main surface of the group III nitridefilm donor substrate.

According to a still further aspect, Reference Invention I provides amethod for manufacturing a group III nitride semiconductor device,including the steps of: preparing a group III nitride compositesubstrate according to the above aspect; and growing at least one groupIII nitride layer on the group III nitride film of the group III nitridecomposite substrate.

The method for manufacturing a group III nitride semiconductor deviceaccording to this aspect of Reference Invention I may further includethe steps of: bonding a device support substrate onto the group IIInitride layer; and removing the support substrate from the group IIInitride composite substrate.

Effects of Reference Invention I

Reference Invention I can provide a low-cost and large-diameter groupIII nitride composite substrate including a group III nitride filmhaving a large thickness, a small thickness variation, and a highcrystal quality, a method for manufacturing the group III nitridecomposite substrate, a laminated group III nitride composite substrate,and a group III nitride semiconductor device and a method formanufacturing the same.

Reference Embodiment I-1: Group III Nitride Composite Substrate

Referring to FIG. 6, a group III nitride composite substrate 1 which isa reference embodiment of Reference Invention I is a group III nitridecomposite substrate 1 with a diameter of 75 mm or more including asupport substrate 11 and a group III nitride film 13 having a thicknessof 10 μm or more and 250 μm or less that are bonded to each other. Theratio s_(t)/m_(t) of a standard deviation s_(t) of the thickness ofgroup III nitride film 13, to a mean value m_(t) of the thicknessthereof is 0.001 or more and 0.2 or less, and the ratio s_(o)/m_(o) of astandard deviation s_(o) of an absolute value of an off angle between amain surface 13 m of group III nitride film 13 and a plane of apredetermined plane orientation, to a mean value m_(o) of the absolutevalue of the off angle thereof is 0.005 or more and 0.6 or less.

Group III nitride composite substrate 1 of the present referenceembodiment has a diameter of 75 mm or more, group III nitride film 13bonded onto support substrate 11 has a thickness of 10 μm or more and250 μm or less, the ratio s_(t)/m_(t) of standard deviation s, of thethickness of group III nitride film 13, to mean value m_(t) of thethickness thereof is 0.001 or more and 0.2 or less, and the ratios_(o)/m_(o) of standard deviation s_(o) of an absolute value of an offangle between main surface 13 m of group III nitride film 13 and a planeof a predetermined plane orientation, to mean value m_(o) of theabsolute value of the off angle thereof is 0.005 or more and 0.6 orless. Accordingly, on group III nitride film 13, at least one group IIInitride layer having a large diameter and a high crystal quality can begrown, and therefore, group III nitride semiconductor devices havingexcellent characteristics can be produced with a high yield.

Regarding group III nitride composite substrate 1 of the presentreference embodiment, the manner in which support substrate 11 and groupIII nitride film 13 are bonded to each other is not particularlylimited. In order to increase the joint strength of the bonding,however, it is preferable to provide a joint film 12 between the supportsubstrate 11 and the group III nitride film 13.

<Diameter of Group III Nitride Composite Substrate>

In order to produce a greater number of semiconductor device chips fromone composite substrate, the diameter of group III nitride compositesubstrate 1 is 75 mm or more, and is preferably 100 mm or more, morepreferably 125 mm or more, and still more preferably 150 mm or more.Meanwhile, in order to reduce the warp of the composite substrate andincrease the yield of semiconductor devices, the diameter of group IIInitride composite substrate 1 is preferably 300 mm or less, and morepreferably 200 mm or less.

<Warp on the Group III Nitride Film Side of Group III Nitride CompositeSubstrate>

In order to increase the yield of semiconductor devices to be formed,group III nitride composite substrate 1 has a warp, on the group IIInitride film 13 side, of preferably 50 μm or less, more preferably 30 μmor less, and still more preferably 20 μm or less. Here, the warp on thegroup III nitride film 13 side of group III nitride composite substrate1 is calculated in the following way. With respect to a least squareplane where the sum of respective squares of the distances from theleast square plane to any points within main surface 13 m of group IIInitride film 13 is a minimum value, the distance between the leastsquare plane and a point on the main surface that is located furthestaway in one direction from the least square plane and the distancebetween the least square plane and a point on the main surface that islocated furthest away in the other direction from the least square planeare added together. The value of this sum represents the warp. The warpis measured by means of an interferometric flatness tester, a laserdisplacement meter, or the like.

<TTV of Group III Nitride Composite Substrate>

In order to increase the yield of semiconductor devices to be formed,the TTV (Total Thickness Variation which is one of indices used forevaluating the flatness and also called GBIR (Global Backside IdealRange)) of group III nitride composite substrate 1 is preferably 30 μmor less, more preferably 20 μm or less, and still more preferably 10 μmor less. Here, the TTV of group III nitride composite substrate 1 is thedifference between the maximum value and the minimum value, within thewhole main surface 13 m of group III nitride film 13, of the distancemeasured in the thickness direction from a reference plane, which is amain surface of support substrate 11 and is the rear surface of groupIII nitride composite substrate 1, to main surface 13 m of group IIInitride film 13, which is the front surface of group III nitridecomposite substrate 1. Specifically, the TTV is the difference betweenthe highest level and the lowest level of the main surface of group IIInitride film 13, which is the front surface of group III nitridecomposite substrate 1, relative to the reference plane, which is themain surface of support substrate 11, namely the rear surface of groupIII nitride composite substrate 1, and corrected to be flat. The leveldifference is measured by means of an interferometric flatness tester, alaser displacement meter, or the like.

<Relation between Support Substrate and Group III Nitride Film in GroupIII Nitride Composite Substrate>

In order to reduce warp and cracks of group III nitride compositesubstrate 1 and of a group III nitride layer to be grown on group IIInitride film 13, the ratio α_(III-N)/α_(S) of thermal expansioncoefficient α_(III-N) of the group III nitride film to thermal expansioncoefficient α_(s) of the support substrate is preferably 0.75 or moreand 1.25 or less, more preferably 0.8 or more and 1.2 or less, stillmore preferably 0.9 or more and 1.1 or less, and particularly preferably0.95 or more and 1.05 or less.

In order to reduce warp and cracks of group III nitride compositesubstrate 1 and of a group III nitride layer to be grown on group IIInitride film 13, the ratio t_(III-N)/t_(S) of thickness t_(III-N) of thegroup III nitride film to thickness t_(S) of the support substrate ispreferably 0.02 or more and 1 or less, more preferably 0.06 or more and0.7 or less, still more preferably 0.15 or more and 0.5 or less, andparticularly preferably 0.2 or more and 0.4 or less.

<Support Substrate>

Support substrate 11 is not particularly limited as long as supportsubstrate 11 can support group III nitride film 13. In order to reducethe amount of use of the expensive group III nitride to thereby lowerthe cost, however, the support substrate is preferably ahetero-composition substrate whose chemical composition is differentfrom that of the group III nitride. Further, since it is preferable asdescribed above that the ratio α_(III-N)/α_(S) of thermal expansioncoefficient α_(III-N) of group III nitride film 13 to thermal expansioncoefficient α_(s) of support substrate 11 is 0.75 or more and 1.25 orless, support substrate 11 is preferably a substrate formed of mullite(3Al₂O₃.2SiO₂-2Al₂O₃.SiO₂), a substrate formed of mullite-YSZ (YttriaStabilized Zirconia), a substrate formed of spinel (MgAl₂O₄), asubstrate formed of a sintered body of an Al₂O₃—SiO₂-based compositeoxide, and substrates formed respectively of sintered bodies of them towhich oxide, carbonate or the like is added, a molybdenum (Mo)substrate, a tungsten (W) substrate, or the like. Here, preferredelements to be contained in the oxide and the carbonate are Ca, Mg, Sr,Ba, Al, Sc, Y, Ce, Pr, Si, Ti, Zr, V, Nb, Ta, Cr, Mn, Fe, Co, Ni, Cu,Zn, and the like.

The root mean square roughness (hereinafter also referred to as RMS) ofmain surface 11 n of support substrate 11 in group III nitride compositesubstrate 1 is preferably 12 nm or less, more preferably 6 nm or less,and still more preferably 2 nm or less, in order to enhance the crystalquality of a group III nitride layer to be grown on group III nitridefilm 13 of group III nitride composite substrate 1. Here, the RMS ofmain surface 11 n of support substrate 11 can be adjusted by polishingboth the main surfaces of support substrate 11 before group III nitridefilm 13 is bonded to support substrate 11, or polishing, after group IIInitride film 13 is bonded to support substrate 11, the other mainsurface to which group III nitride film 13 is not bonded. Here, the RMSof main surface 11 n of support substrate 11 is determined in thefollowing way. From points on main surface 11 n of support substrate 11,a reference plane is calculated, and the positive square root of themean of respective squares of the distances from the aforementionedpoints to the reference plane is determined. The value of the determinedpositive square root is the RMS, which is measured by means of an AFM(Atomic Force Microscope), an interferometric roughness meter, astylus-based roughness meter, or the like.

The main surface 11 n of support substrate 11 preferably has a meanvalue ms of the RMS of 0.3 nm or more and 10 nm or less, and a standarddeviation s_(S) of the RMS of 3 nm or less. In the case where mainsurface 11 n of support substrate 11 has a mean value ms of the RMS of10 nm or less and a standard deviation s_(S) of the RMS of 3 nm or less,a group III nitride layer of high crystal quality can be grown on thewhole main surface 13 m of group III nitride film 13 and accordinglysemiconductor devices can be produced with a high yield. In order formain surface 11 n of support substrate 11 to have a mean value ms of theRMS of less than 0.3 nm, sophisticated surface polishing is necessary,which increases the cost to a considerable degree. In view of the above,mean value ms of the RMS of main surface 11 n of support substrate 11 ismore preferably 0.3 nm or more and 5 nm or less, and still morepreferably 0.3 nm or more and 2 nm or less. In addition, standarddeviation s_(S) of the RMS of main surface 11 n of support substrate 11is more preferably 2 nm or less, and still more preferably 1 nm or less.

Here, mean value ms and standard deviation s_(S) of the RMS of mainsurface 11 n of support substrate 11 are respectively the arithmeticmean and the standard deviation that are calculated from the RMSmeasured at the 13 measurement points on main surface 11 n of supportsubstrate 11 shown in FIG. 7. The 13 measurement points P on mainsurface 11 n of support substrate 11 shown in FIG. 7 are constituted,regardless of the magnitude of the diameter of support substrate 11: onecentral point P_(C); four outer points P_(O) located respectively in thefour directions with respect to central point P_(C) that are orthogonalto each other, the outer points each being located at 5 mm inward fromthe outer edge of the substrate; and eight middle points P_(M) includingfour middle points each between the one central point P_(C) and one ofthe four outer points P_(O) and four middle points each between two ofthe four outer points P_(O). The standard deviation here refers to thepositive square root of unbiased variance.

<Joint Film>

Joint film 12 is not particularly limited as long as joint film 12 canjoin support substrate 11 and group III nitride film 13 to each other.Joint film 12 is preferably SiO₂ film, Si₃N₄ film, TiO₂ film, Ga₂O₃film, or the like because these films have a high joining ability forjoining support substrate 11 and group III nitride film 13 to eachother.

<Group III Nitride Film>

Group III nitride film 13 is a film formed of a group III nitride,specifically an In_(x)Al_(y)Ga_(1-x-y)N film (0≤x, 0≤y, x+y≤1) such asGaN film, AlN film, or the like.

In order to form a group III nitride semiconductor device havingexcellent characteristics, the thickness of group III nitride film 13 is10 μm or more, which is preferably 30 μm or more, more preferably 50 μmor more, and still more preferably 100 μm or more. Meanwhile, in orderto reduce the amount of use of the expensive group III nitride, thethickness of group III nitride film 13 is 250 μm or less, which ispreferably 200 μm or less, and more preferably 170 μm or less, andparticularly preferably 150 μm or less.

The ratio s_(t)/m_(t) of standard deviation s_(t) of the thickness ofgroup III nitride film 13, to mean value m_(t) of the thickness thereofis 0.001 or more and 0.2 or less, which is preferably 0.001 or more and0.15 or less, more preferably 0.002 or more and 0.1 or less, and stillmore preferably 0.01 or more and 0.05 or less. In order to make theratio s_(t)/m_(t) smaller than 0.001, it is necessary to perform cuttingand polishing and thereby precisely control the thickness, whichincreases the cost. On the contrary, if the ratio s_(t)/m_(t) is higherthan 0.2, the uniformity of the film thickness is deteriorated, whichdegrades the characteristics of semiconductor devices to be produced.

The ratio s_(o)/m_(o) of standard deviation s_(o) of an absolute valueof an off angle between main surface 13 m of group III nitride film 13and a plane of a predetermined plane orientation, to mean value m_(a) ofthe absolute value of the off angle thereof is 0.005 or more and 0.6 orless, which is preferably 0.005 or more and 0.5 or less, more preferably0.008 or more and 0.4 or less, and still more preferably 0.05 or moreand 0.2 or less. In order to make the ratio s_(o)/m_(o) smaller than0.005, it is necessary to perform cutting and polishing and therebyprecisely control the off angle, which increases the cost. On thecontrary, if the ratio s_(o)/m_(o) is higher than 0.6, a portion wherethe morphology of a group III nitride layer formed on group III nitridefilm 13 is deteriorated is generated within the main surface. Inaddition, the variation, within the main surface, of the amount ofimpurities taken into the main surface is large, which reduces the yieldof semiconductor devices to be produced.

The crystal structure of group III nitride film 13 is preferably thewurtzite structure, since it enables semiconductor devices havingexcellent characteristics to be produced. The above-referencedpredetermined plane orientation to which main surface 13 m of group IIInitride film 13 is closest is not limited as long as it is suitable fora desired semiconductor device, and may be any of {0001}, {10-10},{11-20}, {21-30}, {20-21}, {10-11}, {11-22}, and {22-43}, as well asplane orientations that are 15° or less off from these planeorientations, respectively. It may also be any of the plane orientationsopposite to the above-listed plane orientations, as well as planeorientations that are 15° or less off from these plane orientations,respectively. Namely, main surface 13 m of group III nitride film 13 maybe any of polar plane, nonpolar plane, and semipolar plane. Main surface13 m of group III nitride film 13 is preferably any of the {0001} planeand the opposite plane thereof, since such planes make it easy toincrease the diameter, or any of {10-10} plane, {20-21} plane, and theopposite planes thereof, since such planes suppress blue shift of alight-emitting device to be produced.

Here, regarding group III nitride film 13, mean value nm of thethickness, standard deviation s_(t) of the thickness, mean value m_(o)of the off angle, and standard deviation s_(o) of the off angle, arevalues of the arithmetic mean and the standard deviation calculated fromrespective measurements of the thickness and respective measurements ofthe off angle that are taken at the 13 measurement points on mainsurface 13 m of group III nitride film 13 shown in FIG. 7. The 13measurement points P on main surface 13 m of group III nitride film 13shown in FIG. 7 are constituted, regardless of the magnitude of thediameter of the group III nitride film: one central point P_(C); fourouter points P_(O) located respectively in the four directions withrespect to central point P_(C) that are orthogonal to each other, theouter points each being located at 5 mm inward from the outer edge; andeight middle points P_(M) including four middle points each between theone central point P_(C) and one of the four outer points P_(O) and fourmiddle points each between two of the four outer points P_(O). Thestandard deviation here refers to the positive square root of unbiasedvariance.

In order to enhance the crystal quality of a group III nitride layer tobe grown on group III nitride film 13 and enhance the characteristics ofsemiconductor devices to be formed, impurity metal atoms of main surface13 m of group III nitride film 13 are preferably 3×10¹² atoms/cm² orless, more preferably 4×10¹¹ atoms/cm² or less, and still morepreferably 1×10¹¹ atoms/cm² or less. As for other impurities of thesurface of group III nitride film 13, Cl atoms are preferably 2×10¹⁴atoms/cm² or less, and Si atoms are preferably 9×10¹³ atoms/cm² or less,in order to enhance the crystal quality of a group III nitride layer tobe grown on group III nitride film 13 and enhance the characteristics ofsemiconductor devices to be formed.

In order to enhance the crystal quality of a group III nitride layer tobe grown on group III nitride film 13, the RMS of main surface 13 m ofgroup III nitride film 13 is preferably 3 nm or less, more preferably 2nm or less, and still more preferably 1 nm or less.

Preferably, main surface 13 m of group III nitride film 13 has a meanvalue m_(III-N) of the RMS of 0.1 nm or more and 2 nm or less, and astandard deviation s_(III-N) of the RMS of 0.4 nm or less. In the casewhere main surface 13 m of group III nitride film 13 has a mean valuem_(III-N) of the RMS of 2 nm or less and a standard deviation s_(III-N)of the RMS of 0.4 nm or less, a group III nitride layer of high crystalquality can be grown on the whole main surface 13 m of group III nitridefilm 13, and accordingly, semiconductor devices can be produced with ahigh yield. In order for main surface 13 m of group III nitride film 13to have a mean value m_(III-N) of the RMS of less than 0.1 nm,sophisticated surface polishing is necessary, which increases the costto a considerable degree. In view of the above, mean value m_(III-N) ofthe RMS of main surface 13 m of group III nitride film 13 is morepreferably 0.1 nm or more and 1.5 nm or less, and still more preferably0.2 nm or more and 1 nm or less. In addition, main surface 13 m of groupIII nitride film 13 has a standard deviation s_(III-N) of the RMS ofmore preferably 0.3 nm or less, and still more preferably 0.2 nm orless.

Here, the mean value m_(III-N) and the standard deviation s_(III-N) ofthe RMS of main surface 13 m of group In nitride film 13 arerespectively the arithmetic mean and the standard deviation that arecalculated from the RMS measured at the 13 measurement points on themain surface of group III nitride film 13 shown in FIG. 7, as describedabove. Moreover, regarding group III nitride film 13, its dislocationdensity is preferably 1×10⁸ cm⁻² or less, and its carrier concentrationis preferably 1×10¹⁷ cm⁻³ or more.

Reference Embodiment I-2: Laminated Group III Nitride CompositeSubstrate

Referring to FIG. 8, a laminated group III nitride composite substrate 2which is another reference embodiment of Reference Invention I includesgroup III nitride composite substrate 1 of Reference Embodiment I-1, andat least one group III nitride layer 20 disposed on group III nitridefilm 13 of group III nitride composite substrate 1.

In laminated group III nitride composite substrate 2 of the presentreference embodiment, group III nitride layer 20 disposed by being grownon group III nitride film 13, which has a high crystal quality as wellas a small thickness variation and a small off-angle variation, also hasa high crystal quality. Therefore, semiconductor devices havingexcellent characteristics can be produced with a high yield.

In laminated group III nitride composite substrate 2 of the presentreference embodiment, group III nitride layer 20 disposed on group IIInitride film 13 varies depending on the type of the semiconductor deviceto be produced. Referring to FIG. 9, in the case where the semiconductordevice to be produced is a light-emitting device, group III nitridelayer 20 may be configured to include, for example, an n-GaN layer 21,an n-In_(0.05)Ga_(0.93)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25. Referring to FIG. 10, in the case where the semiconductordevice to be produced is an HEMT (High Electron Mobility Transistor)which is an example of electronic devices, the group III nitride layermay be configured to include, for example, a GaN layer 26 and anAl_(0.2)Ga_(0.8)N layer 27. Referring to FIG. 11, in the case where thesemiconductor device to be produced is an SBD (Schottky Barrier Diode)which is another example of electronic devices, the group III nitridelayer may be configured to include, for example, an n^(|)-GaN layer 28(having a carrier concentration for example of 2×10¹⁸ cm⁻³) and ann⁻-GaN layer 29 (having a carrier concentration for example of 5×10¹⁵cm⁻³).

Reference Embodiment I-3: Group III Nitride Semiconductor Device

Referring to FIGS. 9 and 10, a group III nitride semiconductor device 4which is still another reference embodiment of Reference Invention Iincludes group III nitride film 13 in the group III nitride compositesubstrate of Reference Embodiment I-1, and at least one group IIInitride layer 20 disposed on group III nitride film 13.

Group III nitride semiconductor device 4 of the present referenceembodiment includes group III nitride film 13 which has a high crystalquality as well as a small thickness variation and a small off-anglevariation, and group III nitride layer 20 of high crystal qualitydisposed by being grown on group III nitride film 13, and therefore hasexcellent characteristics.

In group III nitride semiconductor device 4, group III nitride layer 20varies depending on the type of group III nitride semiconductor device4. Referring to FIG. 9, in the case where group III nitridesemiconductor device 4 is a light-emitting device, group III nitridelayer 20 may be configured to include, for example, an n-GaN layer 21,an n-In_(0.05)Ga_(0.95)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25. Referring to FIG. 10, in the case where group III nitridesemiconductor device 4 is an HEMT which is an example of electronicdevices, group III nitride layer 20 may be configured to include, forexample, a GaN layer 26 and an Al_(0.2)Ga_(0.8)N layer 27. Referring toFIG. 11, in the case where the semiconductor device is an SBD which isanother example of electronic devices, the group III nitride layer maybe configured to include, for example, an n⁺-GaN layer 28 (having acarrier concentration for example of 2×10¹⁸ cm⁻³) and an n⁻-GaN layer 29(having a carrier concentration for example of 5×10¹⁸ cm⁻³).

Preferably, group III nitride semiconductor device 4 further includes atleast one of support substrate 11 and a device support substrate 40 forsupporting group III nitride layer 20. Here, the shape of device supportsubstrate 40 is not limited to the shape of a flat plate, and may be anyas long as it supports group III nitride film 13 and group III nitridelayer 20 so that group III nitride semiconductor device 4 can be formed.

Reference Embodiment I-4: Method for Manufacturing Group III NitrideComposite Substrate

Referring to FIGS. 12 and 13, a method for manufacturing a group IIInitride composite substrate which is a further reference embodiment ofReference Invention I is a method for manufacturing group III nitridecomposite substrate 1 of Reference Embodiment I-1, including the stepsof: forming a joined substrate 1L, 1LS having a diameter of 75 mm ormore by bonding support substrate 11 and a group III nitride film donorsubstrate 13D to each other (FIG. 12 (A) to (C) and FIG. 13 (A) to (C));and forming group III nitride composite substrate 1 by cutting group IIInitride film donor substrate 13D in joined substrate 1L, 1LS along aplane located inwardly at a predetermined distance from the bonded mainsurface of group III nitride film donor substrate 13D (FIG. 12 (D), FIG.13 (D)).

The method for manufacturing a group III nitride composite substrate ofthe present reference embodiment can efficiently manufacture low-costand large-diameter group III nitride composite substrate 1 including agroup III nitride film having a large thickness and a high crystalquality.

Here, in the step of forming group III nitride composite substrate 1,the predetermined distance of the plane located inwardly from the bondedmain surface of group III nitride film donor substrate 13D, for cuttinggroup III nitride film donor substrate 13D along the plane at thisdistance, is determined depending on the thickness of group III nitridefilm 13 of group III nitride composite substrate 1 to be manufactured.

In addition, in the step of forming group III nitride compositesubstrate 1, group III nitride film donor substrate 13D is cut to formgroup III nitride film 13, and thereafter at least one of grinding,polishing, and etching can be performed on the main surface 13 m ofgroup III nitride film 13 opposite to bonded main surface 13 n thereof,to thereby reduce the thickness of group III nitride film 13. Inparticular, in order to reduce the thickness variation and the off-anglevariation of group III nitride film 13 which is formed by cutting groupIII nitride film donor substrate 13D, it is preferable to polish themain surface, which has been obtained by the cutting, of group IIInitride film 13 in group III nitride composite substrate 1. In order toreduce the thickness variation and the off-angle variation of group IIInitride film 13, the method for polishing is preferably precisionpolishing based on CMP (Chemical Mechanical Polishing), chemicalpolishing, or the like.

In view of the above, the predetermined distance of the plane locatedinwardly from bonded main surface 13 n of group III nitride film donorsubstrate 13D, for cutting group III nitride film donor substrate 13Dalong the plane at this distance, is preferably the sum determined byadding a polishing allowance, which is a portion to be removed away bypolishing, to the thickness of group III nitride film 13 of group IIInitride composite substrate 1 to be manufactured.

Regarding the method for manufacturing group III nitride compositesubstrate 1 of the present reference embodiment, group III nitride filmdonor substrate 13D is cut along a plane located inwardly at apredetermined distance from bonded main surface 13 n of group IIInitride film donor substrate 13D in joined substrate 1L, 1LS, andpreferably at least one of grinding, polishing, and etching is performedon the main surface 13 m which is opposite to bonded main surface 13 nof group III nitride film donor substrate 13D, to thereby adjust thefilm thickness by reducing it. Accordingly, group III nitride compositesubstrate 1 including group III nitride film 13 having a desiredthickness of 10 μm or more and 250 μm or less can be obtained.

As for the method for manufacturing group III nitride compositesubstrate 1 of the present reference embodiment, the group III nitridefilm donor substrate is cut in the step of forming the group III nitridecomposite substrate. Thus, in order to improve the workability and theefficiency of manufacture, the thickness of group III nitride film donorsubstrate 13D to be used is preferably larger than 500 μm, morepreferably 1 mm or more, and still more preferably 2 mm or more.

<Step of Forming Joined Substrate>

Referring to FIG. 12 (A) to (C) and FIG. 13 (A) to (C), the step offorming joined substrate 1L, 1LS includes the sub step of forming ajoint film 12 a on a main surface 11 m of support substrate 11 (FIG. 12(A), FIG. 13 (A)), the sub step of forming a joint film 12 b on a mainsurface 13 n of group III nitride film donor substrate 13D (FIG. 12 (B),FIG. 13 (B)), and the sub step of bonding joint film 12 a formed onsupport substrate 1 and joint film 12 b formed on group III nitride filmdonor substrate 13D to each other (FIG. 12 (C), FIG. 13 (C)).

Referring to FIG. 12 (A) and FIG. 13 (A), in the sub step of formingjoint film 12 a on main surface 11 m of support substrate 11, joint film12 a is to be integrated with joint film 12 b, which will be describedlater herein, to form joint film 12, and is made of the same material asjoint film 12. The method for forming joint film 12 a is notparticularly limited as long as the method is suitable for forming jointfilm 12 a. In order to efficiently form joint film 12 a of good quality,however, the method is preferably sputtering, CVD (Chemical VaporDeposition), PLD (Pulsed Laser Deposition), MBE (Molecular BeamEpitaxy), electron-beam deposition, or the like. CVD is particularlypreferred since it enhances the quality of the joint film and enhancesthe film deposition rate. Among different CVD methods, P-CVD(Plasma-Chemical Vapor Deposition), PE-CVD (Plasma Enhanced-ChemicalVapor Deposition), and the like are more preferred since they enable thefilm to be deposited at a low temperature and at a high deposition rate,and LP-CVD (Low Pressure-Chemical Vapor Deposition) and the like arestill more preferred since they enhance the film quality and facilitatemass production.

Further, in order to improve the joint strength, annealing may beperformed after joint films 12 a, 12 b are formed and before they arejoined together. This annealing can degas joint films 12 a, 12 b tothereby densify joint films 12 a, 12 b.

Furthermore, in order to increase the joint strength between supportsubstrate 11 and group III nitride film donor substrate 13D, a mainsurface 12 am of joint film 12 a is preferably mirror-polished (into amirror-finished surface having an RMS of 0.3 nm or less). The method forpolishing main surface 12 am of joint film 12 a is not particularlylimited. For example, CMP (Chemical Mechanical Polishing) or the like isused. In order to improve the cleanliness of the joint film for thepurpose of increasing the joint strength, non-abrasive polishing may beperformed, after CMP, with a solution containing no abrasive. In orderto enhance the effect of removing the abrasive, non-abrasive polishingmay be performed by means of an alkali such as KOH, TMAH(tetramethylammonium hydroxide), or an acid such as HCl, HNO₃, H₂SO₄. Inaddition, in order to improve the cleanliness of the joint film for thepurpose of increasing the joint strength, scrub cleaning using sponge,brush, or the like may be performed. In addition, two-fluid cleaning,megasonic cleaning, ultrasonic cleaning, or the like may suitably beperformed.

Referring to FIG. 12 (B) and FIG. 13 (B), in the sub step of formingjoint film 12 b on main surface 13 n of group III nitride film donorsubstrate 13D, group III nitride film donor substrate 13D is a donorsubstrate which is to provide group III nitride film 13 after a partthereof is separated in the subsequent sub step. The method forpreparing this group III nitride film donor substrate 13D is notparticularly limited. In order to produce group III nitride film donorsubstrate 13D of good crystallinity, suitable methods are gas phasemethods such as HVPE (Hydride Vapor Phase Epitaxy), MOVPE (Metal OrganicVapor Phase Epitaxy), MBE (Molecular Beam Epitaxy), and sublimationmethod, as well as liquid phase methods such as flux method, highnitrogen pressure solution method, ammonothermal method, and the like.Group III nitride film donor substrate 13D prepared in this way is notparticularly limited. In order to provide group III nitride film 13 ofgood crystallinity, the donor substrate preferably has a degree ofcrystallinity substantially identical to that of group III nitride film13 to be provided. Specifically, it is preferable that the ratios_(o)/m_(o) of standard deviation s_(o) of an absolute value of an offangle between main surface 13 m of group III nitride film donorsubstrate 13D and a plane of a predetermined plane orientation, to amean value m_(o) of the absolute value of the off angle thereof is 0.005or more and 0.6 or less.

The material and the method for forming joint film 12 b as well aspolishing of a main surface 12 bn of joint film 12 b are similar to thematerial and the method for forming the above-described joint film 12 aas well as polishing of main surface 12 am of joint film 12 a.

Referring to FIG. 12 (C) and FIG. 13 (C), in the sub step of bondingjoint film 12 a formed on support substrate 11 and joint film 12 bformed on group III nitride film donor substrate 13D to each other, themethod for bonding them together is not particularly limited. Suitablemethods include: a direct bonding method according to which respectivesurfaces to be bonded to each other are cleaned, directly bondedtogether, and thereafter heated to approximately 600° C. to 1200° C. sothat the joint films are joined together; a surface activated bondingmethod according to which the surfaces to be bonded to each other arecleaned, subjected to an activation treatment by means of plasma orions, and thereafter joined together under a low-temperature atmosphereof room temperature (25° C. for example) to 400° C.; a high-pressurebonding method according to which the surfaces to be bonded to eachother are cleaned with a chemical solution and pure water and thereaftersubjected to a high pressure on the order of 0.1 MPa to 10 MPa so thatthe joint films are joined together; a high vacuum bonding methodaccording to which the surfaces to be bonded to each other are cleanedwith a chemical solution and pure water, and thereafter joined togetherunder a high vacuum atmosphere on the order of 10⁻⁶ Pa to 10⁻³ Pa, andthe like. Any of the above-referenced bonding methods can increase thetemperature to approximately 600° C. to 1200° C. after the joint filmsare joined together to thereby further increase the joint strength. Inparticular, the surface activated bonding method, the high pressurebonding method, and the high vacuum bonding method exhibit a highereffect of increasing the joint strength through the heating toapproximately 600° C. to 1200° C. after the joint films are joinedtogether.

The above bonding causes joint film 12 a and joint film 12 b to bejoined together and thereby integrated into joint film 12. Supportsubstrate 11 and group III nitride film donor substrate 13D are joinedtogether with joint film 12 interposed therebetween to thereby formjoined substrate 1L, 1LS.

Respective surfaces of joint films 12 a and 12 b to be bonded to eachother can thus be activated before being bonded to each other, tothereby increase the joint strength. Activation of the surfaces to bebonded is not particularly limited. The surfaces are preferablyactivated through plasma treatment, ion treatment, chemical treatment bymeans of a chemical solution, cleaning, CMP treatment, and the like,since they have a high activation effect.

<Step of Forming Group III Nitride Composite Substrate>

Referring to FIG. 12 (D) and FIG. 13 (D), in the step of forming groupIII nitride composite substrate 1, group III nitride film donorsubstrate 13D is cut along a plane located inwardly at a predetermineddistance from bonded main surface 13 n of group III nitride film donorsubstrate 13D in joined substrate 1L, 1LS. Accordingly, the donorsubstrate is separated into group III nitride film 13, which is joinedto support substrate 11 with joint film 12 interposed therebetween, anda remaining group III nitride film donor substrate 13Dr. Thus, group IIInitride composite substrate 1 in which support substrate 11 and groupIII nitride film 13 are bonded to each other with joint film 12interposed therebetween is formed.

The method for cutting group III nitride film donor substrate 13D is notparticularly limited, and may be any of the methods such as wire saw,blade saw, laser processing, electrical discharge processing, water jet,and the like.

As to cutting of group III nitride film donor substrate 13D by means ofa wire saw, it is preferable to use a fixed-abrasive wire in order toflatly cut group III nitride film donor substrate 13D of a largediameter, and it is preferable to use a thin wire in order to reduce acutting allowance, which is a portion to be removed away throughcutting. For reduction of the cutting allowance, loose-abrasiveprocessing is preferred. In addition, as to cutting of group III nitridefilm donor substrate 13D by means of a wire saw, it is preferable toincrease the tension of the wire and increase the wire speed, in orderto reduce bending of the wire caused by the cut resistance and therebyimprove the thickness precision and the flatness. For this purpose, ahigh-rigidity wire saw apparatus is preferred.

It is also preferable to swing the wire and vibrate group III nitridefilm donor substrate 13D in synchronization therewith, in order toreduce the cut resistance and thereby improve the thickness precisionand the flatness. Specifically, the cutting resistance can be reduced inthe following manner. In the case where the wire saw is positioned at aright angle or an angle close thereto relative to the direction in whichgroup III nitride film donor substrate 13D is cut, group III nitridefilm donor substrate 13D is moved along the direction in which it iscut. In the case where the wire saw is located at an angle further fromthe right angle relative to the direction in which group III nitridefilm donor substrate 13D is cut, group III nitride film donor substrate13D is moved in the direction opposite to the direction in which it iscut.

The group III nitride such as GaN is more brittle and more prone tocrack as compared with sapphire, SiC, and the like, and therefore cannotbe cut appropriately by a cutting method similar to that used forsapphire and SiC. It is therefore necessary for cutting of the group IIInitride to further reduce the cut resistance. In order to reduce the cutresistance and thereby enhance the thickness precision and the flatness,it is preferable that the resistance coefficient R (N) is within anappropriate range, specifically a range of 4000 or more and 5000 orless. The resistance coefficient R (N) is represented by (η×Q×V)/(L×P×n)where η (Pa·s) is the viscosity of a machining fluid for slicing, Q(m³/s) is the flow rate of the machining fluid, V (m/s) is the wirespeed, L (m) is the maximum cut length, P (m/s) is the cut speed, and nis the number of workpieces that are cut simultaneously.

For group III nitride composite substrate 1 obtained through thecutting, respective main surfaces 13 m, 11 n of group III nitride film13 and support substrate 11 can be polished so that a desired thickness,a desired off angle, and the uniformity of them are obtained.Specifically, in order to bond group III nitride composite substrate 1to a polishing apparatus in a polishing process, group III nitridecomposite substrate 1 can be secured by means of suction and/or a backpad. Group III nitride composite substrate 1 may also be bonded first toa holding plate and then bonded to a polishing apparatus. For thisbonding, mechanical pressurization such as vacuum chuck, air backpressurization, weight or the like can be used to reduce inclination andcorrect warp. Group III nitride composite substrate 1 may also besecured by means of suction. Group III nitride composite substrate 1 canuniformly be bonded to the polishing apparatus to reduce the thicknessvariation and the off-angle variation after polishing.

As seen from the above, regarding the method for manufacturing a groupIII nitride composite substrate in the present reference embodiment, itis preferable to polish main surface 13 m of group III nitride film 13in group III nitride composite substrate 1 obtained through cutting, inorder to reduce the thickness variation and the off-angle variation ofgroup III nitride film 13 in group III nitride composite substrate 1,keep the crystal quality high by eliminating a damaged layer caused bycutting of group III nitride film 13, and smooth main surface 13 m.

Accordingly, regarding the method for manufacturing a group III nitridecomposite substrate in the present reference embodiment, thepredetermined distance of the plane located inwardly from bonded mainsurface 13 n of group III nitride film donor substrate 13D, for cuttinggroup III nitride film donor substrate 13D in joined substrate 1L, 1LSalong the plane at this distance, is preferably the sum determined byadding a polishing allowance, which is a portion to be removed away bypolishing, to the thickness of group III nitride film 13 of group IIInitride composite substrate 1 to be manufactured. Here, the polishingallowance is not particularly limited, but preferably 10 μm or more,more preferably 20 μm or more, and still more preferably 30 μm or more,in order to reduce the thickness variation and the off-angle variationand remove a damaged layer. Meanwhile, in order to reduce the materialloss of group III nitride film donor substrate 13D, the polishingallowance is preferably 100 μm or less, more preferably 80 μm or less,and still more preferably 60 μm or less.

Referring also to FIG. 12 (D) and (B) and FIG. 13 (D) and (B), theremaining group III nitride film donor substrate 13Dr may have its mainsurface polished so that it can be used repeatedly.

<Use of Support-Incorporated Group III Nitride Film Donor Substrate>

Referring to FIG. 13 (B) to (D), a support-incorporated group IIInitride film donor substrate 5D in which a group III nitride film donorsubstrate support 15 is bonded to group III nitride film donor substrate13D can be used to produce a group III nitride composite substrate 1 ina similar manner to the above. Support-incorporated group III nitridefilm donor substrate 5D has group III nitride film donor substratesupport 15 which supports group III nitride film donor substrate 13D,and can therefore be used repeatedly even if group III nitride filmdonor substrate 13D becomes thinner to such an extent that substrate 13Dcannot stand by itself.

Regarding support-incorporated group III nitride film donor substrate5D, the form in which group III nitride film donor substrate support 15and group III nitride film donor substrate 13D are bonded to each otheris not particularly limited. In order to increase the joint strength ofthe bonding, however, it is preferable to dispose a joint film 14therebetween. Group III nitride film donor substrate support 15 is alsonot particularly limited. In order to increase the support strength andprevent occurrence of crack and warp, however, it is preferable thatsupport 15 is formed of a material having similar physical properties tosupport substrate 11. While joint film 14 is not particularly limited,it may preferably be any of SiO₂ film, Si₃N₄ film, TiO₂ film, Ga₂O₃film, and the like, since these films provide a good joint to group IIInitride film donor substrate support 15 and group III nitride film donorsubstrate 13D.

Reference Embodiment I-5: Another Method for Manufacturing Group IIINitride Composite Substrate

Referring to FIG. 14, a method for manufacturing a group III nitridecomposite substrate, which is a still further reference embodiment ofReference Invention I, is specifically a method for manufacturing groupIII nitride composite substrate 1 of the Reference Embodiment I-1,including the steps of: forming joined substrate 1L having a diameter of75 mm or more by bonding support substrate 11 and group III nitride filmdonor substrate 13D to each other (FIG. 14 (A) to (C)); and forminggroup III nitride composite substrate 1 by performing at least one ofgrinding, polishing, and etching on a main surface of group III nitridefilm donor substrate 13D of joined substrate 1L, the main surface beinglocated opposite to the bonded main surface of group III nitride filmdonor substrate 13D (FIG. 14 (D)).

In accordance with the method for manufacturing a group III nitridecomposite substrate in the present reference embodiment, low-cost andlarge-diameter group III nitride composite substrate 1 including a groupIII nitride film having a large thickness and a high crystal quality canefficiently be manufactured.

Regarding the method for manufacturing group III nitride compositesubstrate 1 in the present reference embodiment, at least one ofgrinding, polishing, and etching is performed on the main surfaceopposite to the bonded main surface of group III nitride film donorsubstrate 13D to thereby adjust the film thickness by reducing it, andaccordingly, group III nitride composite substrate 1 including group IIInitride film 13 having a desired thickness of 10 μm or more and 250 μmless can be obtained.

According to the method for manufacturing group III nitride compositesubstrate 1 in the present reference embodiment, in the step of forminga group III nitride composite substrate, at least one of grinding,polishing, and etching is performed on the main surface opposite to thebonded main surface of the group III nitride film donor substrate.Therefore, in order to reduce the material loss of group III nitridefilm donor substrate 13D, the thickness of group III nitride film donorsubstrate 13D to be used is preferably 500 μm or less, and morepreferably 400 μm or less.

<Step of Forming Joined Substrate>

Referring to FIG. 14 (A) to (C), the step of forming joined substrate 1Lincludes, similarly to the method for manufacturing a group III nitridecomposite substrate in Reference Embodiment I-4, the sub step of formingjoint film 12 a on main surface 11 m of support substrate 11 (FIG. 14(A)), the sub step of forming joint film 12 b on main surface 13 n ofgroup III nitride film donor substrate 13D (FIG. 14 (B)), and the stepof bonding joint film 12 a formed on support substrate 11 and joint film12 b formed on group III nitride film donor substrate 13D to each other(FIG. 14 (C)).

Here, the sub step of forming joint film 12 a on main surface 11 m ofsupport substrate 11 as shown in FIG. 14 (A) is similar to the sub stepof forming joint film 12 a on main surface 11 m of support substrate 11as shown in FIG. 12 (A). The sub step of forming joint film 12 b on mainsurface 13 n of group III nitride film donor substrate 13D as shown inFIG. 14 (B) is similar to the sub step of forming joint film 12 b onmain surface 13 n of group III nitride film donor substrate 13D as shownin FIG. 12 (B). The sub step of bonding joint film 12 a formed onsupport substrate 11 and joint film 12 b formed on group III nitridefilm donor substrate 13D to each other as shown in FIG. 14 (C) issimilar to the sub step of bonding joint film 12 a formed on supportsubstrate 11 and joint film 12 b formed on group III nitride film donorsubstrate 13D to each other as shown in FIG. 12 (C). Therefore, thedescription of them will not be repeated.

<Step of Forming Group III Nitride Composite Substrate>

Referring to FIG. 14 (D), in the step of forming group III nitridecomposite substrate 1, at least one of grinding, polishing, and etchingis performed on main surface 13 m opposite to bonded main surface 13 nof group III nitride film donor substrate 13D in joined substrate 1L, tothereby form group III nitride film 13 with its thickness reduced, fromgroup III nitride film donor substrate 13D, and accordingly form groupIII nitride composite substrate 1 in which support substrate 11 andgroup III nitride film 13 are bonded together with joint film 12interposed therebetween.

The method for grinding group III nitride film donor substrate 13D isnot particularly limited, and may be grinding by means of one of agrinding wheel and abrasive grains. The method for polishing group IIInitride film donor substrate 13D is not particularly limited, and may berough polishing such as mechanical polishing, precision polishing suchas CMP and chemical polishing, or the like. The method for etching groupIII nitride film donor substrate 13D is not particularly limited, andmay be wet etching using a chemical solution, dry etching such as RIE(Reactive Ion Etching), or the like.

In order to reduce the thickness variation and the off-angle variationof group III nitride film 13 to be formed, it is preferable to polishmain surface 13 m of group III nitride film 13 of group III nitridecomposite substrate 1 obtained through at least one of grinding andetching. The reduction of the thickness by at least one of grinding,polishing, and etching is preferably 10 μm or more, more preferably 20μm or more, and still more preferably 30 μm or more, in order to reducethe thickness variation and the off-angle variation, and remove adamaged layer. Meanwhile, the reduction of the thickness by at least oneof grinding, polishing, and etching is preferably 100 μm or less, morepreferably 80 μm or less, and still more preferably 60 μm or less, inorder to reduce the material loss of group III nitride film donorsubstrate 13D.

Reference Embodiment I-6: Method for Manufacturing Group III NitrideSemiconductor Device

Referring to FIG. 15, a method for manufacturing a group III nitridesemiconductor device, which is a still further reference embodiment ofReference Invention I, includes the steps of: preparing group IIInitride composite substrate 1 of Reference Embodiment I-1; and growingat least one group III nitride layer 20 on group III nitride film 13 ofgroup III nitride composite substrate 1 (FIG. 15 (A)). The method formanufacturing a group III nitride semiconductor device in the presentreference embodiment can be used to manufacture group III nitridesemiconductor devices having excellent characteristics with a highyield.

The method for manufacturing a group III nitride semiconductor device inthe present reference embodiment may further include the steps of:bonding a device support substrate 40 onto group III nitride layer 20(FIG. 15 (B)); and removing support substrate 11 from group III nitridecomposite substrate 1 (FIG. 15 (C)). These steps can be added tomanufacture, with a high yield, group III nitride semiconductor devicessupported by device support substrate 40 and having a high mechanicalstrength and excellent characteristics.

The method for manufacturing a group III nitride semiconductor device inthe present reference embodiment may specifically be performed throughthe following steps.

<Step of Growing Group III Nitride Layer>

Referring to FIG. 15 (A), in the step of growing at least one group IIInitride layer 20 on group III nitride film 13 of group III nitridecomposite substrate 1, suitable methods for growing group III nitridelayer 20 are gas phase methods such as MOVPE, MBE, HVPE, and sublimationmethod, as well as liquid phase methods such as flux method, in order toepitaxially grow group III nitride layer 20 having a high crystalquality, and a particularly suitable method is MOVPE.

The structure of group III nitride layer 20 varies depending on the typeof group III nitride semiconductor device 4. In the case where group IIInitride semiconductor device 4 is a light-emitting device, group IIInitride layer 20 may be configured by successively growing, on group IIInitride film 13, for example, an n-GaN layer 21, ann-In_(0.05)Ga_(0.95)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25.

In this way, at least one group III nitride layer 20 is grown on groupIII nitride film 13 of group III nitride composite substrate 1, andaccordingly laminated group III nitride composite substrate 2 isobtained.

<Step of Bonding Device Support Substrate>

Referring to FIG. 15 (B), the step of bonding device support substrate40 onto group III nitride layer 20 is performed by forming a firstelectrode 30 and a pad electrode 33 on group III nitride layer 20 oflaminated group III nitride composite substrate 2, forming a padelectrode 43 and a joint metal film 44 on device support substrate 40,and bonding joint metal film 44 to pad electrode 33. Through thesesteps, laminated substrate 3 is obtained. As device support substrate40, Si substrate, CuW substrate, or the like is used.

<Step of Removing Support Substrate>

Referring to FIG. 15 (C), the step of removing support substrate 11 fromgroup III nitride composite substrate 1 is performed by removing supportsubstrate 11 of group III nitride composite substrate 1 from laminatedsubstrate 3. In the case where group III nitride composite substrate 1includes joint film 12 interposed between support substrate 11 and groupIII nitride film 13, joint film 12 can also be removed.

The method for removing support substrate 11 and joint film 12 is notparticularly limited, and suitable methods to be used are grinding,etching, and the like. For example, support substrate 11 formed of amaterial which is low in hardness, strength, and wear resistance, and islikely to be ground off, can be removed by at least one of grinding andpolishing, in order to reduce the manufacturing cost. Support substrate11 formed of a material which can be dissolved in a chemical solutionsuch as acid solution or alkali solution, can be removed through etchingwith a chemical solution, since it requires low manufacturing cost. Assupport substrate 11, a support substrate formed of a polycrystallinematerial such as ceramic material is more preferred as compared with asupport substrate formed of a monocrystalline material such as sapphire,SiC, group III nitride (such as GaN), or the like, since the supportsubstrate 11 of the polycrystalline material is easier to remove.

<Step of Forming Electrode>

Referring to FIG. 15 (D), on group III nitride film 13 which has beenexposed after removal of support substrate 11 and joint film 12 fromlaminated substrate 3, a second electrode 50 is formed and, on devicesupport substrate 40, a device support substrate electrode 45 is formed.

<<Reference Invention II>>

Reference Invention II relates to a low-cost and large-diameter groupIII nitride composite substrate which includes a thick group III nitridefilm and has a small temperature variation on a main surface when agroup III nitride layer is grown, to enable reduction of the cost ofmanufacturing a semiconductor device and enable group III nitridesemiconductor devices to be manufactured with a high yield. It alsorelates to a method for manufacturing the group III nitride compositesubstrate, a laminated group III nitride composite substrate, as well asa group III nitride semiconductor device and a method for manufacturingthe same.

[Background Art Regarding Reference Invention II]

Group III nitrides such as GaN have superior semiconductor propertiesand are therefore used as materials suitable for semiconductor devices.In addition, group III nitrides such as GaN have excellent materialcharacteristics different from those of Si, GaAs, SiC, or the like fordifferent types of semiconductor devices such as light-emitting deviceand electronic device for example, and can therefore be used for avariety of devices.

For example, Japanese Patent Laying-Open No. 2009-126722 discloses afree-standing group III nitride substrate to be used as a substrate fora semiconductor device. The free-standing group III nitride substratehas a diameter of 25 mm or more and 160 mm or less and a thickness of100 μm or more and 1000 μm or less. It discloses, as a specific examplethereof, a free-standing GaN substrate having a diameter of 100 mm and athickness of 400 μm.

Japanese Patent Laying-Open No. 2008-010766 discloses aGaN-thin-film-bonded substrate to be used as a substrate formanufacturing a semiconductor device. The GaN-thin-film-bonded substrateincludes a heterogeneous substrate whose chemical composition isdifferent from that of GaN, and a GaN thin film having a thickness of0.1 μm or more and 100 μm or less and bonded to the heterogeneoussubstrate. It discloses, as a specific example thereof, aGaN-thin-film-bonded substrate having a diameter of 50.8 mm andincluding a sapphire substrate and a GaN thin film having a thickness of0.1 μm or 100 μm and bonded to the sapphire substrate.

Japanese Patent Laying-Open No. 2010-182936 discloses a compositesubstrate to be used as a substrate for a semiconductor device. Thecomposite substrate includes a support substrate, a nitridesemiconductor layer, and a joint layer disposed between the supportsubstrate and the nitride semiconductor layer. It discloses, as aspecific example thereof, a composite substrate having a diameter of50.8 mm and including a sapphire substrate, a GaN layer, and a jointlayer formed by press fitting between the substrate and the GaN layer,in which the GaN layer has a thickness of 5 μm to 220 μm.

Problems to be Solved by Reference Invention II

The free-standing group III nitride substrate disclosed in JapanesePatent Laying-Open No. 2009-126722 involves problems that the substrateis manufactured at high cost and therefore very expensive, and that thesubstrate is likely to crack, resulting in difficulty in increasing thediameter of the substrate and decreasing the thickness thereof. Further,it is necessary to reduce the thickness of the free-standing group IIInitride substrate when a semiconductor device is to be formed, which isaccompanied by a problem of an increase in manufacturing cost due to aprocess step such as grinding of the rear surface (a main surfaceopposite to a main surface on which a group III nitride layer is formedfor implementing device functions, and this definition of the rearsurface is applied hereinafter) of the free-standing group III nitridesubstrate.

Regarding the GaN-thin-film-bonded substrate having the GaN thin film of0.1 μm in thickness as disclosed in Japanese Patent Laying-Open No.2008-010766, ion implantation is performed to form the GaN thin film,which, however, involves a problem that the ion implantation degradesthe crystal quality of the GaN thin film. In order to enhance thecharacteristics of the semiconductor device to be formed, the thicknessof the GaN thin film is preferably 10 μm or more. An increase inthickness of the GaN thin film, however, involves a problem that thevariation in depth to which ions are implanted from the main surfaceincreases, which accordingly increases the variation in thickness of theGaN thin film in the resultant GaN-thin-film-bonded substrate.

Moreover, the GaN-thin-film composite substrate disclosed in JapanesePatent Laying-Open No. 2008-010766 and the composite substrate disclosedin Japanese Patent Laying-Open No. 2010-182936 are each a compositesubstrate in which a film or layer of group III nitride is joined onto asupport substrate, and are therefore likely to have a large temperaturevariation on the main surface of the substrate when the group IIInitride layer is formed on the substrate for manufacturing a group IIInitride semiconductor device, as compared with the free-standing groupIII nitride substrate disclosed in Japanese Patent Laying-Open No.2009-126722. Accordingly, the above-referenced composite substratessuffer from a problem of difficulty in increasing the yield ofsemiconductor devices to be manufactured, if the diameter of thesubstrate is increased.

An object of Reference Invention II is to solve the above problems andthereby provide a low-cost and large-diameter group III nitridecomposite substrate which includes a thick group III nitride film andhas a small temperature variation (namely unevenness, and thisdefinition of the variation is applied hereinafter) on a main surfacewhen a group III nitride layer is grown, to enable reduction of the costof manufacturing a semiconductor device and enable group III nitridesemiconductor devices to be manufactured with a high yield, and providea method for manufacturing the group III nitride composite substrate, alaminated group III nitride composite substrate, as well as a group IIInitride semiconductor device and a method for manufacturing the same.

Solution to the Problems

According to an aspect, Reference Invention II provides a group IIInitride composite substrate with a diameter of 75 mm or more including asupport substrate and a group III nitride film having a thickness of 10μm or more and 250 μm or less that are bonded to each other, a meanvalue ms of a root mean square roughness of a support-substrate-sidemain surface being 0.3 nm or more and 20 nm or less, and a ratios_(S)/m_(S) of a standard deviation s_(S) of the root mean squareroughness, to the mean value ms of the root mean square roughness of thesupport-substrate-side main surface, being 0.005 or more and 0.4 orless.

Regarding the group III nitride composite substrate according to theabove aspect of Reference Invention II, a mean value m_(III-N) of a rootmean square roughness of a group III-nitride-film-side main surface maybe 0.15 nm or more and 3 nm or less, and a ratio s_(III-N)/m_(III-N) ofa standard deviation s_(III-N) of the root mean square roughness, to themean value m_(III-N) of the root mean square roughness of the groupIII-nitride-film-side main surface, may be 0.008 or more and 0.5 orless. A ratio W/D of a warp W of the support-substrate-side main surfaceto the diameter D may be −7×10⁻⁴ or more and 8×10⁴ or less. A ratioα_(III-N)/α_(S) of a thermal expansion coefficient α_(III-N) of thegroup III nitride film to a thermal expansion coefficient α_(S) of thesupport substrate may be 0.75 or more and 1.25 or less, and a ratiot_(III-N)/t_(S) of a thickness t_(III-N) of the group III nitride filmto a thickness t_(S) of the support substrate may be 0.02 or more and 1or less. The support substrate may have a thermal conductivity λ_(S) of3 W·m⁻¹·K⁻¹ or more and 280 W·m⁻¹·K⁻¹ or less. The support substrate mayhave a Young's modulus E_(S) of 150 GPa or more and 500 GPa or less. Thediameter may be 100 mm or more. Further, the diameter may be 125 mm ormore and 300 mm or less.

According to another aspect, Reference Invention II provides a laminatedgroup III nitride composite substrate including the group III nitridecomposite substrate according to the above aspect, and at least onegroup III nitride layer disposed on the group III-nitride-film-side mainsurface of the group III nitride composite substrate.

According to still another aspect, Reference Invention 11 provides agroup III nitride semiconductor device including the group III nitridefilm in the group III nitride composite substrate according to the aboveaspect, and at least one group III nitride layer disposed on the groupIII nitride film.

According to a further aspect, Reference Invention II provides a methodfor manufacturing a group III nitride composite substrate according tothe above aspect, including the steps of: forming a joined substratewith a diameter of 75 mm or more by bonding a support substrate and agroup III nitride film donor substrate to each other; forming the groupIII nitride composite substrate by cutting the group III nitride filmdonor substrate in the joined substrate along a plane located inwardlyat a predetermined distance from a bonded main surface of the group IIInitride film donor substrate; and adjusting the root mean squareroughness of the support-substrate-side main surface by polishing thesupport-substrate-side main surface of the group III nitride compositesubstrate before, in, or after any one of the above steps.

According to a still further aspect, Reference Invention 11 provides amethod for manufacturing a group III nitride composite substrateaccording to the above aspect, including the steps of: forming a joinedsubstrate with a diameter of 75 mm or more by bonding a supportsubstrate and a group III nitride film donor substrate to each other;forming the group III nitride composite substrate by performing at leastone of grinding, polishing, and etching on a main surface of the groupIII nitride film donor substrate in the joined substrate, the mainsurface being opposite to a bonded main surface of the group III nitridefilm donor substrate; and adjusting the root mean square roughness ofthe support-substrate-side main surface by polishing thesupport-substrate-side main surface of the group III nitride compositesubstrate before, in, or after any one of the above steps.

According to a still further aspect, Reference Invention II provides amethod for manufacturing a group III nitride semiconductor device,including the steps of: preparing a group III nitride compositesubstrate according to the above aspect; and growing at least one groupIII nitride layer on a group III-nitride-film-side main surface of thegroup III nitride composite substrate.

The method for manufacturing a group III nitride semiconductor deviceaccording to this aspect of Reference Invention II may further includethe step of removing the support substrate from the group III nitridecomposite substrate, after the step of growing the group III nitridelayer. It may further include the step of bonding a device supportsubstrate onto the group III nitride layer, after the step of growingthe group n nitride layer and before the step of removing the supportsubstrate.

[Effects of Reference Invention II]

Reference Invention II can provide a low-cost and large-diameter groupIII nitride composite substrate which includes a thick group III nitridefilm and has a small temperature variation on a main surface when agroup III nitride layer is grown, to enable reduction of the cost ofmanufacturing a semiconductor device and enable group III nitridesemiconductor devices to be manufactured with a high yield, and providea method for manufacturing the group III nitride composite substrate, alaminated group III nitride composite substrate, as well as a group IIInitride semiconductor device and a method for manufacturing the same.

Reference Embodiment II-1: Group III Nitride Composite Substrate

Referring to FIG. 6, a group III nitride composite substrate 1 which isa reference embodiment of Reference Invention II is a group III nitridecomposite substrate 1 with a diameter of 75 mm or more including asupport substrate 11 and a group III nitride film 13 having a thicknessof 10 μm or more and 250 μm or less that are bonded to each other, amean value ms of a root mean square roughness (hereinafter also referredto as RMS) of a support-substrate 11-side main surface 11 n, which is amain surface on the support-substrate side, is 0.3 nm or more and 20 nmor less, and a ratio s_(S)/m_(S) of a standard deviation s_(S) of theRMS, to the mean value ms of the RMS of support-substrate 11-side mainsurface 11 n, is 0.005 or more and 0.4 or less.

Regarding group I nitride composite substrate 1 in the present referenceembodiment, the temperature variation (namely unevenness) on the mainsurface is small when a group III nitride layer is grown formanufacturing a group III nitride semiconductor device, and therefore,the group III nitride layer having a high crystal quality can be grown.Thus, high-quality group III nitride semiconductor devices can bemanufactured with a high yield. This will be described in more detail inthe following.

Referring to FIGS. 6 and 8, group III nitride composite substrate 1 inthe present reference embodiment has a structure in which group IIInitride film 13 is bonded onto support substrate 11. When a group IIInitride semiconductor device is manufactured, a susceptor (not shown)including a temperature raising apparatus is arranged so thatsupport-substrate 11-side main surface 11 n, which corresponds to therear side of group III nitride composite substrate 1, is opposite to amain surface of the susceptor, and at least one group III nitride layer20 is grown on a group III-nitride-film 13-side main surface 13 m, whichcorresponds to the front side of group III nitride composite substrate1.

Group III nitride composite substrate 1 in the present referenceembodiment has a diameter of 75 mm or more, group III nitride film 13bonded onto support substrate 11 of the composite substrate has athickness of 10 μm or more and 250 μm or less, mean value ms of the RMSof support-substrate 11-side main surface 11 n is 0.3 nm or more and 20nm or less, and a ratio s_(S)/m_(S) of standard deviation s_(S) of theRMS, to mean value ms of the RMS of support-substrate 11-side mainsurface 11 n is 0.005 or more and 0.4 or less. Thus, heat is uniformlytransmitted from the main surface of the susceptor whose temperature hasbeen raised, to the whole of support-substrate 11-side main surface 11 nwhich is the rear side of large-diameter group III nitride compositesubstrate 1 and has a small average and variation of the RMS. Therefore,group III nitride composite substrate 1 with a large diameter isentirely heated uniformly. Accordingly, a small and uniform temperaturevariation on the group III-nitride-film 13-side main surface 13 m, whichis the front side of group III nitride composite substrate 1 of a largediameter, is obtained. Thus, on group III-nitride-film 13-side mainsurface 13 m of large-diameter group III nitride composite substrate 1,uniform and large-diameter group III nitride layer 20 having a highcrystal quality can be grown, and therefore, group III nitridesemiconductor devices having excellent characteristics can bemanufactured with a high yield.

Referring to FIG. 6, the manner in which support substrate 11 and groupIII nitride film 13 are bonded to each other in group III nitridecomposite substrate 1 is not particularly limited. In order to increasethe joint strength of the bonding, however, it is preferable to providea joint film 12 between the support substrate and the group III nitridefilm.

<Thickness of Group III Nitride Film>

Referring to FIGS. 6 and 7, the thickness of group III nitride film 13of group III nitride composite substrate 1 in the present referenceembodiment is 10 μm or more and 250 μm or less. Here, the thickness ofgroup III nitride film 13 refers to the mean value calculated from thethicknesses measured at 13 measurement points on main surface 13 m ofgroup III nitride film 13 shown in FIG. 7. The 13 measurement points Pon the main surface of group III nitride film 13 shown in FIG. 7 areconstituted, regardless of the magnitude of the diameter of the groupIII nitride film: one central point P_(C); four outer points P_(O)located respectively in the four directions with respect to centralpoint P_(C) that are orthogonal to each other, the outer points eachbeing located at 5 mm inward from the outer edge of the substrate; andeight middle points P_(M) including four middle points each between theone central point P_(C) and one of the four outer points P_(O) and fourmiddle points each between two of the four outer points P_(O).

In order to grow a group III nitride layer having a high crystalquality, group III nitride film 13 needs a thickness of 10 μm or more,preferably 30 μm or more, more preferably 80 μm or more, and still morepreferably 100 μm or more. Meanwhile, in order to reduce the cost of thematerial for group III nitride composite substrate 1, group III nitridefilm 13 needs a thickness of 250 μm or less, preferably 200 μm or less,more preferably 180 or less, and still more preferably 130 μm or less.

<Diameter of Group III Nitride Composite Substrate>

Referring to FIG. 6, group III nitride composite substrate 1 in thepresent reference embodiment has a diameter of 75 mm or more. In orderto produce a greater number of semiconductor device chips from onecomposite substrate, group III nitride composite substrate 1 needs adiameter of 75 mm or more, preferably 100 mm or more, more preferably125 mm or more, and still more preferably 150 mm or more. Meanwhile, inorder to reduce the warp of the composite substrate and increase theyield of semiconductor devices, the diameter of group III nitridecomposite substrate 1 is preferably 300 mm or less, and more preferably200 mm or less.

<RMS of Support-Substrate-Side Main Surface>

Referring to FIGS. 6 and 7, group III nitride composite substrate 1 inthe present reference embodiment has a mean value ms of the RMS ofsupport-substrate 11-side main surface 11 n of 0.3 nm or more and 20 nmor less, and a ratio s_(S)/m_(S) of standard deviation s_(S) of the RMSto mean value ms of the RMS of support-substrate 11-side main surface 11n of 0.005 or more and 0.4 or less.

Regarding support-substrate 11-side main surface 11 n which is the rearside of group III nitride composite substrate 1, in order to grow auniform group III nitride layer having a high quality on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1, support-substrate 11-side main surface 11 n needsa mean value ms of the RMS of 20 nm or less, preferably 10 nm or less,and more preferably 5 nm or less, and a ratio s_(S)/m_(S) of standarddeviation s_(S) of the RMS to mean value ms of the RMS of 0.4 or less,preferably 0.3 or less, and more preferably 0.2 or less.

Regarding support-substrate 11-side main surface 11 n which is the rearside of group III nitride composite substrate 1, in order to reduce thecost of surface treatment of main surface 11 n, support-substrate11-side main surface 1 in needs a mean value ms of the RMS of 0.3 nm ormore, preferably 0.5 nm or more, and more preferably 1 nm or more, and aratio s_(S)/m_(S) of standard deviation s_(S) of the RMS to mean valuems of the RMS of 0.005 or more, preferably 0.01 or more, and morepreferably 0.05 or more.

Referring to FIG. 7, mean value ms and standard deviation s_(S) of theRMS of support-substrate 11-side main surface 1 in of group III nitridecomposite substrate 1 are respectively the arithmetic mean and thestandard deviation that are calculated from the RMS measured at the 13measurement points P on main surface 11 n of support substrate 11. The13 measurement points P on main surface 11 n of support substrate 11shown in FIG. 7 are constituted, regardless of the magnitude of thediameter of support substrate 11: one central point P_(C); four outerpoints P_(O) located respectively in the four directions with respect tocentral point P_(C) that are orthogonal to each other, the outer pointseach being located at 5 mm inward from the outer edge of the substrate;and eight middle points P_(M) including four middle points each betweenthe one central point P_(C) and one of the four outer points P_(O) andfour middle points each between two of the four outer points P_(O). Thestandard deviation refers to the positive square root of unbiasedvariance.

The RMS measured at the 13 measurement points P on main surface 11 n ofsupport substrate 11 shown in FIG. 7 refers to the positive square rootof the mean of respective squares of the distances to the points from areference plane which is calculated from each point in a measurementregion having a size of 85 μm×85 μm in which measurement point P islocated at the center, and is measured by means of an AFM (Atomic ForceMicroscope), an interferometric roughness meter, a laser microscope, astylus-based roughness meter, or the like.

It should be noted that mean value m_(s) and standard deviation s_(S) ofthe RMS of support-substrate 11-side main surface 11 n of group IIInitride composite substrate 1 can be adjusted, as described laterherein, by physical properties of abrasives, a surface plate, and apolishing pad for polishing main surface 11 n of support substrate 11,the shapes of the surface plate and the polishing pad, and polishingconditions.

<RMS of Group III-Nitride-Film-Side Main Surface>

Referring to FIGS. 6 and 7, group III nitride composite substrate 1 inthe present reference embodiment preferably has a mean value m_(III-N)of the RMS of group III-nitride-film 13-side main surface 13 m of 0.15nm or more and 3 nm or less, and preferably has a ratios_(III-N)/m_(III-N) of standard deviation s_(III-N) of the RMS to meanvalue m_(III-N) of the RMS of group III-nitride-film 13-side mainsurface 13 m of 0.008 or more and 0.5 or less.

Regarding group III-nitride-film 13-side main surface 13 m which is thefront side of group III nitride composite substrate 1, in order to growa uniform group III nitride layer having a high crystal quality on mainsurface 13 m, mean value m_(III-N) of the RMS is preferably 3 nm orless, more preferably 2 nm or less, and still more preferably 1.6 nm orless, and ratio s_(III-N)/m_(III-N) of standard deviation s_(III-N) ofthe RMS to mean value m_(III-N) of the RMS is preferably 0.5 or less,more preferably 0.4 or less, and still more preferably 0.2 or less.

Further, regarding group III-nitride-film 13-side main surface 13 mwhich is the front side of group III nitride composite substrate 1, inorder to reduce the cost of surface treatment of main surface 13 m, meanvalue m_(III-N) of the RMS is preferably 0.15 nm or more, morepreferably 0.4 nm or more, and still more preferably 0.8 nm or more, andratio s_(III-N)/m_(III-N) of standard deviation s_(III-N) of the RMS tomean value m_(III-N) of the RMS is preferably 0.008 or more, morepreferably 0.02 or more, and still more preferably 0.05 or more.

Referring to FIG. 7, mean value m_(III-N) and standard deviations_(III-N) of the RMS of group III-nitride-film 13-side main surface 13 mof group III nitride composite substrate 1 are respectively thearithmetic mean and the standard deviation that are calculated from theRMS measured at the 13 measurement points P on main surface 13 m ofgroup III nitride film 13. The 13 measurement points P on main surface13 m of group III nitride film 13 shown in FIG. 7 are constituted,regardless of the magnitude of the diameter of group III nitride film13: one central point P_(C); four outer points P_(O) locatedrespectively in the four directions with respect to central point P_(C)that are orthogonal to each other, the outer points each being locatedat 5 mm inward from the outer edge of the substrate; and eight middlepoints P_(M) including four middle points each between the one centralpoint P_(C) and one of the four outer points P_(O) and four middlepoints each between two of the four outer points P_(O). The standarddeviation refers to the positive square root of unbiased variance.

The RMS measured at the 13 measurement points P on main surface 13 m ofgroup III nitride film 13 shown in FIG. 7 refers to the positive squareroot of the mean of respective squares of the distances to the pointsfrom a reference plane which is calculated from each point in ameasurement region having a size of 85 μm×85 μm in which measurementpoint P is located at the center, and is measured by means of an AFM(Atomic Force Microscope), an interferometric roughness meter, a lasermicroscope, a stylus-based roughness meter, or the like.

It should be noted that mean value m_(III-N) and standard deviations_(III-N) of the RMS of group III-nitride-film 13-side main surface 13 mof group III nitride composite substrate 1 can be adjusted, as describedlater herein, by physical properties of abrasives, a surface plate, anda polishing pad for polishing main surface 13 m of group III nitridefilm 13, the shapes of the surface plate and the polishing pad, andpolishing conditions.

<Ratio of Warp of Support-Substrate-Side Main Surface to Diameter>

Referring to FIG. 6, regarding group III nitride composite substrate 1in the present reference embodiment, a ratio W/D of a warp W ofsupport-substrate 11-side main surface 11 n to a diameter D ispreferably −7×10⁻⁴ or more and 8×10⁻⁴ or less, more preferably −4×10⁻⁴or more and 5×10⁻⁴ or less, still more preferably −2.5×10⁻⁴ or more and3×10⁻⁴ or less, and particularly preferably −1×10⁻⁴ or more and 1.5×10⁻⁴or less. Regarding the signs for warp W and ratio W/D, a concave warp ofsupport-substrate 11-side main surface 11 n is herein indicated with the+(positive) sign, and a convex warp of support-substrate 11-side mainsurface 11 n is herein indicated with the — (negative) sign. If ratioW/D of warp W of support-substrate 11-side main surface 11 n to diameterD of group III nitride composite substrate 1 is a small value ofpreferably −7×10⁻⁴ or more and 8×10⁻⁴ or less, more preferably −4×10⁴ ormore and 5×10⁻⁴ or less, still more preferably −2.5×10⁻⁴ or more and3×10⁻⁴ or less, and particularly preferably −1×10⁻⁴ or more and 1.5×10⁻⁴or less, heat is uniformly transmitted to the whole of support-substrate11-side main surface 11 n of large-diameter group III nitride compositesubstrate 1, from the main surface of the susceptor with its temperatureincreased, when a group III nitride layer is grown on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1, and therefore, the whole of group III nitridecomposite substrate 1 is uniformly heated. Accordingly, the temperatureon group III-nitride-film 13-side main surface 13 m which is the frontside of group III nitride composite substrate 1 has a small variationand uniformity. Thus, a uniform and large-diameter group III nitridelayer having a high crystal quality can be grown on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1. Accordingly, group III nitride semiconductordevices having excellent characteristics can be manufactured with a highyield.

<Ratio of Thermal Expansion Coefficient of Group III Nitride Film toThermal Expansion Coefficient of Support Substrate>

Referring to FIG. 6, regarding group III nitride composite substrate 1in the present reference embodiment, in order to suppress warp andcracks of group III nitride composite substrate 1 and of a group IIInitride layer grown on group III nitride film 13 and thereby increasethe yield of group III nitride semiconductor devices, a ratioα_(III-N)/α_(S) of a thermal expansion coefficient α_(III-N) of groupIII nitride film 13 to a thermal expansion coefficient α_(S) of supportsubstrate 1 is preferably 0.75 or more and 1.25 or less, more preferably0.85 or more and 1.15 or less, and still more preferably 0.95 or moreand 1.05 or less. Thermal expansion coefficient α_(S) of supportsubstrate 11 and thermal expansion coefficient α_(III-N) of group IIInitride film 13 can be measured with a thermomechanical analysisapparatus.

<Ratio of Thickness of Group III Nitride Film to Thickness of SupportSubstrate>

Referring to FIG. 6, regarding group III nitride composite substrate 1in the present reference embodiment, in order to suppress warp andcracks of group III nitride composite substrate 1 and of a group IIInitride layer grown on group III nitride film 13 and thereby increasethe yield of group III nitride semiconductor devices, a ratiot_(III-N)/t_(S) of a thickness t_(III-N) of group III nitride film 13 toa thickness t_(S) of support substrate 11 is preferably 0.02 or more and1 or less, more preferably 0.1 or more and 0.6 or less, and still morepreferably 0.2 or more and 0.4 or less. Thickness t_(S) of supportsubstrate 11 and thickness t_(III-N) of group III nitride film 13 can bemeasured through observation of a cross section of the film with anoptical microscope and/or an SEM or with a digital indicator, or thelike.

Thus, in order to suppress warp and cracks of group III nitridecomposite substrate 1 and a group III nitride layer grown on group IIInitride film 13 and thereby increase the yield of group III nitridesemiconductor devices, ratio α_(III-N)/α_(S) of thermal expansioncoefficient α_(III-N) of group III nitride film 13 to thermal expansioncoefficient as of support substrate 11 is preferably 0.75 or more and1.25 or less, and ratio t_(III-N)/t_(S) of a thickness t_(III-N) of thegroup III nitride film to thickness t_(S) of support substrate 11 ispreferably 0.02 or more and 1 or less. It is more preferable that ratioα_(III-N)/α_(S) is 0.85 or more and 1.15 or less and ratiot_(III-N)/t_(S) is 0.1 or more and 0.6 or less, and it is furtherpreferable that ratio α_(III-N)/α_(S) is 0.95 or more and 1.05 or lessand ratio t_(III-N)/t_(S) is 0.2 or more and 0.4 or less.

<Support Substrate>

Support substrate 11 included in group III nitride composite substrate 1in the present reference embodiment is not particularly limited as longas support substrate 11 can support group III nitride film 13. In orderto reduce the amount of use of the expensive group III nitride tothereby lower the cost, however, the support substrate is preferably ahetero-composition substrate whose chemical composition is differentfrom that of the group III nitride.

As described above, group III nitride composite substrate 1 in thepresent reference embodiment preferably has a ratio α_(III-N)/α_(S) ofthermal expansion coefficient α_(III-N) of group III nitride film 13 tothermal expansion coefficient α_(s) of support substrate 11 of 0.75 ormore and 1.25 or less.

Regarding group III nitride composite substrate 1 in the presentreference embodiment, thermal conductivity λ_(S) of support substrate 11is preferably 3 W·m⁻¹·K⁻¹ or more and 280 W·m⁻¹·K⁻¹ or less, morepreferably 5 W·m⁻¹·K⁻¹ or more and 210 W·m⁻¹·K⁻¹ or less, and still morepreferably 10 W·m⁻¹·K⁻¹ or more and 120 W·m⁻¹·K⁻¹ or less. Here, thermalconductivity λ_(s) of support substrate 1 can be measured by a laserflash method. Group III nitride composite substrate 1 having supportsubstrate 11 with a thermal conductivity λ_(S) of preferably 3 W·m⁻¹·K⁻¹or more, more preferably 5 W·m⁻¹·K⁻¹ or more, and still more preferably10 W·m⁻¹·K⁻¹ or more, can efficiently transmit heat from the mainsurface of the susceptor to main surface 13 m of group III nitride film13 of group III nitride composite substrate 1, when a group III nitridelayer is grown. Group III nitride composite substrate 1 having supportsubstrate 11 with a thermal conductivity Is of preferably 280 W·m⁻¹·K⁻¹or less, more preferably 210 W·m⁻¹·K⁻¹ or less, still more preferably120 W·m⁻¹·K⁻¹ or less, and particularly preferably 50 W·m⁻¹·K⁻¹ or less,can uniformly transmit heat from the main surface of the susceptor tothe whole main surface of group III nitride film 13 of group III nitridecomposite substrate 1, when a group III nitride layer is grown. Supportsubstrate 11 having a thermal conductivity λ_(S) of 280 W·m⁻¹·K⁻¹ orless can more uniformly transmit the heat from the main surface of thesusceptor to the whole main surface of group III nitride film 13 ofgroup III nitride composite substrate 1 when a group III nitride layeris grown, as compared with the case where an SiC substrate having athermal conductivity λ_(S) of about 300 W·m⁻¹·K⁻¹ is used as the supportsubstrate.

Regarding group III nitride composite substrate 1 in the presentreference embodiment, support substrate 11 has a Young's modulus E_(S)of preferably 150 GPa or more and 500 GPa or less, and more preferably200 GPa or more and 350 GPa or less. Here, Young's modulus E_(S) ofsupport substrate 11 can be measured by a resonance method. Group IIInitride composite substrate 1 having support substrate 11 with a Young'smodulus E_(S) of preferably 150 GPa or more and more preferably 200 GPaor more can suppress occurrence of warp to group III nitride compositesubstrate 1 and/or a group III nitride layer, when a group III nitridesemiconductor device is formed by growing the group III nitride layer onthe composite substrate. Group III nitride composite substrate 1 havingsupport substrate 11 with a Young's modulus E_(S) of preferably 500 GPaor less and more preferably 350 GPa or less can suppress occurrence ofwarp and/or cracks to group III nitride composite substrate 1 and/or agroup III nitride layer, when a group III nitride semiconductor deviceis formed by growing the group III nitride layer on the compositesubstrate.

Support substrate 11 is not particularly limited. In view of theabove-described respects, however, support substrate 11 preferablysatisfies at least one of: ratio α_(III-N)/α_(S) of thermal expansioncoefficient α_(III-N) of group III nitride film 13 to thermal expansioncoefficient α_(S) of support substrate 11 is 0.75 or more and 1.25 orless; support substrate 11 has a thermal conductivity λ_(S) of 3W·m⁻¹·K⁻¹ or more and 280 W·m⁻¹·K⁻¹ or less; and support substrate 11has a Young's modulus E_(S) of 150 GPa or more and 500 GPa or less.Support substrate 11 is preferably a substrate formed of mullite(3Al₂O₃.2SiO₂-2Al₂O₃.SiO₂), a substrate formed of mullite-YSZ (YttriaStabilized Zirconia), a substrate formed of spinel (MgAl₂O₄), asubstrate formed of a sintered body of an Al₂O₃—SiO₂-based compositeoxide, and substrates formed respectively of sintered bodies of them towhich oxide, carbonate or the like is added, a molybdenum (Mo)substrate, a tungsten (W) substrate, or the like. Here, preferredelements to be contained in the oxide and the carbonate are Ca, Mg, Sr,Ba, Al, Sc, Y, Ce, Pr, Si, Ti, Zr, V, Nb, Ta, Cr, Mn, Fe, Co, Ni, Cu,Zn, and the like.

Support substrate 11 may include any of single crystal, polycrystal, andamorphous material. It preferably includes polycrystal, since removal bygrinding or etching when a device is to be formed is easy and thestrength by which warp or cracks can be suppressed can be maintained.

<Joint Film>

Referring to FIG. 6, a joint film 12 which may be included in group IIInitride composite substrate 1 in the present reference embodiment is notparticularly limited as long as the joint film can join supportsubstrate 11 and group III nitride film 13 to each other. Joint film 12,however, is preferably SiO₂ film, Si₃N₄ film, TiO₂ film, Ga₂O₃ film, orthe like because these films have a high joining ability for joiningsupport substrate 11 and group III nitride film 13 to each other.

<Group III Nitride Film>

Referring to FIG. 6, group III nitride film 13 in the present referenceembodiment is a film formed of a group III nitride, specifically anIn_(x)Al_(y)Ga_(1-x-y)N film (0≤x, 0≤y, x+y≤1) such as GaN film, AlNfilm, or the like.

In order to form a group ITT nitride semiconductor device havingexcellent characteristics, the thickness of group III nitride film 13needs to be 10 μm or more, preferably 30 μm or more, more preferably 80μm or more, and still more preferably 100 μm or more, as describedabove. Meanwhile, in order to reduce the amount of use of the expensivegroup III nitride, the thickness of group III nitride film 13 is 250 μmor less, preferably 200 μm or less, more preferably 180 μm or less, andparticularly preferably 130 μm or less, as described above.

The crystal structure of group III nitride film 13 is preferably thewurtzite structure, since it enables semiconductor devices havingexcellent characteristics to be produced. The predetermined planeorientation to which the main surface of group III nitride film 13 isclosest is not limited as long as it is suitable for a desiredsemiconductor device, and may be any of {0001}, {10-10}, {11-20},{21-30}, {20-21}, {10-11}, {11-22}, and {22-43}, as well as planeorientations that are 15° or less off (displaced by an angle of 15° orless) from these plane orientations, respectively. It may also be any ofthe plane orientations opposite to the above-listed plane orientations,as well as plane orientations that are 15° or less off from these planeorientations, respectively. Namely, main surface 13 m of group IIInitride film 13 may be any of polar plane, nonpolar plane, and semipolarplane. Main surface 13 m of group III nitride film 13 is preferably the{0001} plane and the opposite plane thereof, since such planes make iteasy to increase the diameter, or any of {10-10} plane, {20-21} plane,and the opposite planes thereof, since such planes suppress blue shiftof a light-emitting device to be produced.

In order to enhance the crystal quality of a group III nitride layer tobe grown on group III nitride film 13 and enhance the characteristics ofsemiconductor devices to be formed, impurity metal atoms of main surface13 m of group III nitride film 13 are preferably 1×10¹³ atoms/cm² orless, more preferably 3×10¹² atoms/cm² or less, still more preferably1×10¹² atoms/cm² or less, and particularly preferably 1×10¹¹ atoms/cm²or less.

Group III nitride composite substrate 1 which includes support substrate11 such as a substrate of mullite (3Al₂O₃.2SiO₂-2Al₂O₃.SiO₂),mullite-YSZ (Yttria Stabilized Zirconia), spinel (MgAl₂O₄), a sinteredbody of an Al₂O₃—SiO₂-based composite oxide, or the like is preferablysubjected to cleaning that suppresses elution of metal atoms fromsupport substrate 11, such as scrub cleaning with a surfactant and/orpure water, two-fluid cleaning, or megasonic cleaning (cleaning withultrasonic waves in a megasonic frequency range of 500 kHz to 5 MHz), aswell as cleaning of a single side (the single side is main surface 13 mof group III nitride film 13) such as sheet-fed cleaning withlow-concentration acid and/or alkali, for example, to thereby reduce theconcentration of impurity metal atoms in main surface 13 m of group IIInitride film 13. A protective film can also be formed on the supportsubstrate to suppress elution of metal atoms.

Regarding impurities other than impurity metal atoms in main surface 13m of group I nitride film 13, in order to enhance the crystal quality ofa group III nitride layer grown on group III nitride film 13 and enhancethe characteristics of a semiconductor device to be formed, theimpurities are preferably Cl atoms of 2×10¹⁴ atoms/cm² or less,preferably Si atoms of 9×10¹³ atoms/cm² or less. The dislocation densityof group III nitride film 13 is not particularly limited. In order toreduce leak current of the semiconductor device, the dislocation densityis preferably 1×10⁸ cm⁻² or less, and more preferably 1×10⁷ cm⁻² orless. The carrier concentration of group III nitride film 13 is notparticularly limited. In order to reduce the resistance of thesemiconductor device, the carrier concentration is preferably 1×10¹⁷cm⁻¹ or more, and more preferably 1×10¹⁸ cm⁻³ or more.

Reference Embodiment II-2: Laminated Group III Nitride CompositeSubstrate

Referring to FIG. 8, a laminated group III nitride composite substrate 2which is another reference embodiment of Reference Invention 11 includesgroup III nitride composite substrate 1 of Reference Embodiment II-1,and at least one group III nitride layer 20 disposed on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1.

Laminated group III nitride composite substrate 2 in the presentreference embodiment includes group III nitride composite substrate 1having a small mean value ms and a small standard deviation s_(S) of theRMS of support-substrate 11-side main surface 11 n, as well as group IIInitride layer 20 having a high crystal quality grown and therebydisposed on composite substrate 1. Therefore, semiconductor deviceshaving excellent characteristics can be produced with a high yield.

In laminated group III nitride composite substrate 2 of the presentreference embodiment, group III nitride layer 20 disposed on groupIII-nitride-film 13-side main surface 13 m varies depending on the typeof the semiconductor device to be produced.

Referring to FIG. 9, in the case where the semiconductor device to beproduced is a light-emitting device, group III nitride layer 20 may beconfigured to include, for example, an n-GaN layer 21, ann-In_(0.05)Ga_(0.95)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25.

Referring to FIG. 10, in the case where the semiconductor device to beproduced is an HEMT (High Electron Mobility Transistor) which is anexample of electronic devices, group III nitride layer 20 may beconfigured to include, for example, a GaN layer 26 and anAl_(0.2)Ga_(0.8)N layer 27. Referring to FIG. 11, in the case where thesemiconductor device to be produced is an SBD (Schottky Barrier Diode)which is another example of electronic devices, group III nitride layer20 may be configured to include, for example, an n⁺-GaN layer 28 (havinga carrier concentration for example of 2×10¹⁸ cm⁻³) and an n⁻-GaN layer29 (having a carrier concentration for example of 5×10¹⁵ cm⁻³).

Reference Embodiment II-3: Group III Nitride Semiconductor Device

Referring to FIGS. 9 to 11, a group III nitride semiconductor device 4which is still another reference embodiment of Reference Invention IIincludes group III nitride film 13 in the group III nitride compositesubstrate of Reference Embodiment II-1, and at least one group IIInitride layer 20 disposed on group III nitride film 13.

Group III nitride semiconductor device 4 of the present referenceembodiment includes group III nitride film 13 in group III nitridecomposite substrate 1 having a small mean value ms and a small standarddeviation s_(S) of the RMS of support-substrate 11-side main surface 11n, as well as group III nitride layer 20 having a high crystal qualitygrown and thereby disposed on composite substrate 1. Therefore,semiconductor device 4 has excellent characteristics.

In group III nitride semiconductor device 4, group III nitride layer 20varies depending on the type of group III nitride semiconductor device4. Referring to FIG. 9, in the case where group III nitridesemiconductor device 4 is a light-emitting device, group III nitridelayer 20 may be configured to include, for example, an n-GaN layer 21,an n-In_(0.05)Ga_(0.95)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25. Referring to FIG. 10, in the case where group III nitridesemiconductor device 4 is an HEMT which is an example of electronicdevices, group III nitride layer 20 may be configured to include, forexample, a GaN layer 26 and an Al_(0.2)Ga_(0.8)N layer 27. Referring toFIG. 11, in the case where group III nitride semiconductor device 4 isan SBD which is another example of electronic devices, group III nitridelayer 20 may be configured to include, for example, an n⁺-GaN layer 28(having a carrier concentration for example of 2×10¹⁸ cm⁻³) and ann⁻-GaN layer 29 (having a carrier concentration for example of 5×10¹⁵cm⁻³). Other examples of electronic devices may include PND (PN diode),transistor, and the like. The above-described semiconductor devices mayeither be a vertical type or be a lateral type.

Referring to FIG. 9, preferably group III nitride semiconductor device 4further includes at least one of support substrate 11 and a devicesupport substrate 40 for supporting group III nitride layer 20. Here,the shape of device support substrate 40 is not limited to the shape ofa flat plate, and may be any as long as it supports group III nitridefilm 13 and group III nitride layer 20 so that group III nitridesemiconductor device 4 can be formed.

Reference Embodiment III-4: Method for Manufacturing Group III NitrideComposite Substrate Reference Embodiment II-4-1: A Method forManufacturing Group III Nitride Composite Substrate

Referring to FIGS. 12 and 13, a method for manufacturing a group IIInitride composite substrate which is a further reference embodiment ofReference Invention II is a method for manufacturing group III nitridecomposite substrate 1 of Reference Embodiment II-1, including the stepsof: forming a joined substrate 1L, 1LS having a diameter of 75 mm ormore by bonding support substrate 11 and a group III nitride film donorsubstrate 13D to each other (FIG. 12 (A) to (C) and FIG. 13 (A) to (C));forming group III nitride composite substrate 1 by cutting group IIInitride film donor substrate 13D in joined substrate 1L, 1LS along aplane located inwardly at a predetermined distance from the bonded mainsurface of group III nitride film donor substrate 13D (FIG. 12 (D), FIG.13 (D)); and adjusting the RMS of support-substrate 11-side main surface11 n by polishing support-substrate 11-side main surface 11 n of groupIII nitride composite substrate 1 before, in, or after any one of theabove-described steps.

The method for manufacturing group III nitride composite substrate 1 inthe present reference embodiment can efficiently manufacture low-costand large-diameter group III nitride composite substrate 1 having athick group III nitride film and a small temperature variation on themain surface when a group III nitride layer is grown, to enable groupIII nitride semiconductor devices to be manufactured with a high yield.

Here, in the step of forming group III nitride composite substrate 1,the predetermined distance of the plane located inwardly from the bondedmain surface of group III nitride film donor substrate 13D, for cuttinggroup III nitride film donor substrate 13D along the plane at thisdistance, is determined depending on the thickness of group III nitridefilm 13 of group III nitride composite substrate 1 to be manufactured.

In addition, in the step of forming group III nitride compositesubstrate 1, group III nitride film donor substrate 13D is cut to formgroup III nitride film 13, and thereafter at least one of grinding,polishing, and etching can be performed on the main surface 13 m ofgroup III nitride film 13 opposite to bonded main surface 13 n thereof,to thereby reduce the thickness of group III nitride film 13. Inparticular, in order to reduce the thickness variation (namelyunevenness) of group III nitride film 13 which is formed by cuttinggroup III nitride film donor substrate 13D, it is preferable to polishthe main surface of group III nitride film 13 in group III nitridecomposite substrate 1 obtained by cutting. In order to reduce thethickness variation of group III nitride film 13, the method forpolishing is preferably precision polishing based on CMP (ChemicalMechanical Polishing), chemical polishing, or the like.

In view of the above respects, the predetermined distance of the planelocated inwardly from bonded main surface 13 n of group III nitride filmdonor substrate 13D, for cutting group III nitride film donor substrate13D along the plane at this distance, is preferably the sum determinedby adding a polishing allowance, which is a portion to be removed awayby polishing, to the thickness of group III nitride film 13 of group IIInitride composite substrate 1 to be manufactured.

Regarding the method for manufacturing group III nitride compositesubstrate 1 of the present reference embodiment, group III nitride filmdonor substrate 13D is cut along a plane located inwardly at apredetermined distance from bonded main surface 13 n of group IIInitride film donor substrate 13D in joined substrate 1L, 1LS, andpreferably at least one of grinding, polishing, and etching is performedon the main surface 13 m which is opposite to bonded main surface 13 nof group III nitride film 13, to thereby adjust the film thickness byreducing it. Accordingly, group III nitride composite substrate 1including group III nitride film 13 having a desired thickness of 10 μmor more and 250 μm or less can be obtained.

As for the method for manufacturing group III nitride compositesubstrate 1 of the present reference embodiment, in order to improve theworkability and the efficiency of cutting group III nitride film donorsubstrate 13D in the step of forming the group III nitride compositesubstrate, the thickness of group III nitride film donor substrate 13Dto be used is preferably larger than 500 μm, more preferably 1 mm ormore, and still more preferably 2 mm or more.

<Step of Forming Joined Substrate>

Referring to FIG. 12 (A) to (C) and FIG. 13 (A) to (C), the step offorming joined substrate 1L, 1LS includes the sub step of forming ajoint film 12 a on a main surface 11 m of support substrate 11 (FIG. 12(A), FIG. 13 (A)), the sub step of forming a joint film 12 b on a mainsurface 13 n of group III nitride film donor substrate 13D (FIG. 12 (B),FIG. 13 (B)), and the sub step of bonding a main surface 12 am of jointfilm 12 a formed on main surface 11 m of support substrate 11 and a mainsurface 12 bn of joint film 12 b formed on main surface 13 n of groupIII nitride film donor substrate 13D to each other (FIG. 12 (C), FIG. 13(C)).

Referring to FIG. 12 (A) and FIG. 13 (A), in the sub step of formingjoint film 12 a on main surface 11 m of support substrate 11, joint film12 a is to be integrated with joint film 12 b, which will be describedlater herein, to form joint film 12, and is made of the same material asjoint film 12. The method for forming joint film 12 a is notparticularly limited as long as the method is suitable for forming jointfilm 12 a. In order to efficiently form joint film 12 a of good quality,however, the method is preferably sputtering, CVD (Chemical VaporDeposition), PLD (Pulsed Laser Deposition), MBE (Molecular BeamEpitaxy), electron-beam deposition, or the like.

CVD is particularly preferred since it enhances the quality of the jointfilm and enhances the film deposition rate. Among different CVD methods,P-CVD (Plasma-Chemical Vapor Deposition), PE-CVD (PlasmaEnhanced-Chemical Vapor Deposition), and the like are more preferredsince they enable the film to be deposited at a low temperature and at ahigh deposition rate, LP-CVD (Low Pressure-Chemical Vapor Deposition)and the like are still more preferred since they enhance the filmquality and facilitate mass production, and AP-CVD (AtmosphericPressure-Chemical Vapor Deposition) and the like are still morepreferred since they enable the film to be deposited at a higherdeposition rate and they are excellent in productivity.

Further, in order to improve the joint strength, annealing may beperformed after joint films 12 a, 12 b are formed and before they arejoined together. This annealing can degas joint films 12 a, 12 b tothereby densify joint films 12 a, 12 b.

Furthermore, in order to increase the joint strength between supportsubstrate 11 and group III nitride film donor substrate 13D, mainsurface 12 am of joint film 12 a is preferably mirror-polished (into amirror-finished surface having an RMS of 0.3 nm or less). The method forpolishing main surface 12 am of joint film 12 a is not particularlylimited. For example, CMP (Chemical Mechanical Polishing) or the like isused. In order to improve the cleanliness of the joint film for thepurpose of increasing the joint strength, non-abrasive polishing may beperformed, after CMP, with a solution containing no abrasive. In orderto enhance the effect of removing the abrasive, non-abrasive polishingmay be performed by means of an alkali such as KOH, TMAH(tetramethylammonium hydroxide), or an acid such as HCl, HNO₃, H₂SO₄. Inaddition, in order to improve the cleanliness of the joint film for thepurpose of increasing the joint strength, scrub cleaning using sponge,brush, or the like may be performed. In addition, two-fluid cleaning,megasonic cleaning, ultrasonic cleaning, or the like may suitably beperformed.

Referring to FIG. 12 (B) and FIG. 13 (B), in the sub step of formingjoint film 12 b on main surface 13 n of group III nitride film donorsubstrate 13D, group III nitride film donor substrate 13D is a donorsubstrate which is to provide group III nitride film 13 in thesubsequent sub step. The method for preparing this group III nitridefilm donor substrate 13D is not particularly limited. In order toproduce group III nitride film donor substrate 13D of a high crystalquality, suitable methods are gas phase methods such as HVPE (HydrideVapor Phase Epitaxy), MOVPE (Metal Organic Vapor Phase Epitaxy), MBE(Molecular Beam Epitaxy), and sublimation method, as well as liquidphase methods such as flux method, high nitrogen pressure solutionmethod, ammonothermal method, and the like. Group III nitride film donorsubstrate 13D prepared in this way is not particularly limited. In orderto provide group III nitride film 13 of a high crystal quality, thedonor substrate preferably has a degree of crystal quality substantiallyidentical to that of group III nitride film 13 to be provided.

The material and the method for forming joint film 12 b as well aspolishing of a main surface 12 bn of joint film 12 b are similar to thematerial and the method for forming the above-described joint film 12 aas well as polishing of main surface 12 am of joint film 12 a.

Referring to FIG. 12 (C) and FIG. 13 (C), in the sub step of bondingmain surface 12 am of joint film 12 a formed on main surface 11 m ofsupport substrate 11 and main surface 12 bn of joint film 12 b formed onmain surface 13 n of group III nitride film donor substrate 13D to eachother, the method for bonding them together is not particularly limited.Suitable methods include: a direct bonding method according to whichrespective surfaces to be bonded to each other are cleaned, directlybonded together, and thereafter heated to approximately 600° C. to 1200°C. so that the joint films are joined together; a surface activatedbonding method according to which the surfaces to be bonded to eachother are cleaned, subjected to an activation treatment by means ofplasma or ions, and thereafter joined together under a low-temperatureatmosphere of room temperature (25° C. for example) to 400° C.; ahigh-pressure bonding method according to which the surfaces to bebonded to each other are cleaned with a chemical solution and pure waterand thereafter subjected to a high pressure on the order of 0.1 MPa to10 MPa so that the joint films are joined together; a high vacuumbonding method according to which the surfaces to be bonded to eachother are cleaned with a chemical solution and pure water, andthereafter joined together under a high vacuum atmosphere on the orderof 10⁻⁶ Pa to 10⁻³ Pa, and the like. Any of the above-referenced bondingmethods can increase the temperature to approximately 600° C. to 1200°C. after the joint films are joined together to thereby further increasethe joint strength. In particular, the surface activated bonding method,the high pressure bonding method, and the high vacuum bonding methodexhibit a higher effect of increasing the joint strength through theheating to approximately 600° C. to 1200° C. after the joint films arejoined together.

The above bonding causes joint film 12 a and joint film 12 b to bejoined together and thereby integrated into joint film 12. Supportsubstrate 11 and group III nitride film donor substrate 13D are joinedtogether with joint film 12 interposed therebetween to thereby formjoined substrate 1L, 1LS.

Respective main surfaces 12 am and 12 bn of joint films 12 a and 12 b tobe bonded to each other can thus be activated before being bonded toeach other, to thereby increase the joint strength. Activation of mainsurfaces 12 am and 12 bn to be bonded is not particularly limited. Thesurfaces are preferably activated through plasma treatment, iontreatment, chemical treatment by means of a chemical solution, cleaning,CMP treatment, and the like, since they have a high activation effect.

<Step of Forming Group III Nitride Composite Substrate>

Referring to FIG. 12 (D) and FIG. 13 (D), in the step of forming groupIII nitride composite substrate 1, group III nitride film donorsubstrate 13D is cut along a plane located inwardly at a predetermineddistance from bonded main surface 13 n of group III nitride film donorsubstrate 13D in joined substrate 1L, 1LS. Accordingly, the donorsubstrate is separated into group III nitride film 13, which is joinedto support substrate 11 with joint film 12 interposed therebetween, anda remaining group III nitride film donor substrate 13Dr. Thus, group IIInitride composite substrate 1 in which support substrate 11 and groupIII nitride film 13 are bonded to each other with joint film 12interposed therebetween is formed.

The method for cutting group III nitride film donor substrate 13D is notparticularly limited, and may be any of the methods such as wire saw,blade saw, laser processing, electrical discharge processing, water jet,and the like.

As to cutting of group III nitride film donor substrate 13D by means ofa wire saw, it is preferable to use a fixed-abrasive wire in order toflatly cut group III nitride film donor substrate 13D of a largediameter, and it is preferable to use a thin wire in order to reduce acutting allowance, which is a portion to be removed away throughcutting. For reduction of the cutting allowance, loose-abrasiveprocessing is preferred.

In addition, as to cutting of group III nitride film donor substrate 13Dby means of a wire saw, it is preferable to increase the tension of thewire and increase the wire speed, in order to reduce bending of the wirecaused by the cut resistance and thereby improve the thickness precisionand the flatness. For this purpose, a high-rigidity wire saw apparatusis preferred.

It is also preferable to swing the wire and vibrate group III nitridefilm donor substrate 13D in synchronization therewith, in order toreduce the cut resistance and thereby improve the thickness precisionand the flatness. Specifically, the cutting resistance can be reduced inthe following manner. In the case where the wire saw is positioned at aright angle or an angle close thereto relative to the direction in whichgroup III nitride film donor substrate 13D is cut, group III nitridefilm donor substrate 13D is moved along the direction in which it iscut. In the case where the wire saw is located at an angle further fromthe right angle relative to the direction in which group III nitridefilm donor substrate 13D is cut, group III nitride film donor substrate13D is moved in the direction opposite to the direction in which it iscut.

The group III nitride such as GaN is more brittle and more prone tocrack as compared with sapphire, SiC, and the like, and therefore cannotbe cut appropriately by a cutting method similar to that used forsapphire and SiC. It is therefore necessary for cutting of the group IIInitride to further reduce the cut resistance. In order to reduce the cutresistance and thereby enhance the thickness precision and the flatness,it is preferable that the resistance coefficient R (unit: N) is withinan appropriate range, specifically a range of 4000 N or more and 5000 Nor less. The resistance coefficient R (N) is represented byR=(η×Q×V)/(L×P×n) where n (unit: Pa·s) is the viscosity of a machiningfluid for slicing, Q (unit: m³/s) is the flow rate of the machiningfluid, V (unit: m/s) is the wire speed, L (unit: m) is the maximum cutlength, P (unit: m/s) is the cut speed, and n is the number ofworkpieces that are cut simultaneously.

For group III nitride composite substrate 1 obtained through thecutting, respective main surfaces of group III nitride film 13 andsupport substrate 11 can be polished so that a desired thickness and theuniformity of them are obtained. Specifically, in order to bond groupIII nitride composite substrate 1 to a polishing apparatus in apolishing process, group III nitride composite substrate 1 can besecured by means of suction or a back pad. Group III nitride compositesubstrate 1 may also be bonded first to a holding plate and then bondedto a polishing apparatus. For this bonding, mechanical pressurizationsuch as vacuum chuck, air back pressurization, weight or the like can beused to reduce inclination and correct warp. Group III nitride compositesubstrate 1 may also be secured by means of suction. Group III nitridecomposite substrate 1 can uniformly be bonded to the polishing apparatusto reduce the thickness variation after polishing. In addition, theshape of bonding can be controlled depending on the shape of a polishingsurface plate and the shape of the pad to thereby reduce the thicknessvariation after polishing.

As seen from the above, regarding the method for manufacturing a groupIII nitride composite substrate in the present reference embodiment, itis preferable to polish main surface 13 m of group III nitride film 13in group III nitride composite substrate 1 obtained through cutting, inorder to reduce the thickness variation of group III nitride film 13 ingroup III nitride composite substrate 1, keep the crystal quality highby eliminating a damaged layer caused by cutting of group III nitridefilm 13, and smooth the main surface.

Accordingly, regarding the method for manufacturing a group III nitridecomposite substrate in the present reference embodiment, thepredetermined distance of the plane located inwardly from bonded mainsurface 13 n of group III nitride film donor substrate 13D, for cuttinggroup III nitride film donor substrate 13D in joined substrate 1L, 1LSalong the plane at this distance, is preferably the sum determined byadding a polishing allowance, which is a portion to be removed away bypolishing, to the thickness of group III nitride film 13 of group IIInitride composite substrate 1 to be manufactured. Here, the polishingallowance is not particularly limited, but preferably 10 μm or more,more preferably 20 μm or more, and still more preferably 30 μm or more,in order to reduce the thickness variation and remove a damaged layer.

Meanwhile, in order to reduce the material loss of group III nitridefilm donor substrate 13D, the polishing allowance is preferably 100 μmor less, more preferably 80 μm or less, and still more preferably 60 μmor less.

Referring also to FIG. 12 (D) and (B) and FIG. 13 (D) and (B), theremaining group III nitride film donor substrate 13Dr may have its mainsurface polished so that it can be used repeatedly.

<Use of Support-Incorporated Group III Nitride Film Donor Substrate>

Referring to FIG. 13 (B) to (D), a support-incorporated group IIInitride film donor substrate 5D in which a group III nitride film donorsubstrate support 15 is bonded to group III nitride film donor substrate13D can be used to produce a group III nitride composite substrate 1 ina similar manner to the above. Support-incorporated group III nitridefilm donor substrate 5D has group III nitride film donor substratesupport 15 which supports group III nitride film donor substrate 13D,and can therefore be used repeatedly even if group III nitride filmdonor substrate 13D becomes thinner to such an extent that substrate 13Dcannot stand by itself.

Regarding support-incorporated group III nitride film donor substrate5D, the form in which group III nitride film donor substrate support 15and group III nitride film donor substrate 13D are bonded to each otheris not particularly limited. In order to increase the joint strength ofthe bonding, however, it is preferable to dispose a joint film 14therebetween. Group III nitride film donor substrate support 15 is alsonot particularly limited. In order to increase the support strength andprevent occurrence of crack and warp, however, it is preferable thatsupport 15 is formed of a material having similar physical properties tosupport substrate 11. While joint film 14 is not particularly limited,it may preferably be any of SiO₂ film, Si₃N₄ film, TiO₂ film, Ga₂O₃film, and the like, since these films provide a good joint to group IIInitride film donor substrate support 15 and group III nitride film donorsubstrate 13D.

<Step of Adjusting Root Mean Square Roughness of Support-Substrate-SideMain Surface of Group III Nitride Composite Substrate>

The method for manufacturing group IT nitride composite substrate 1 inthe present reference embodiment includes the step of adjusting the RMSof support-substrate 11-side main surface 11 n by polishingsupport-substrate 11-side main surface 11 n of group III nitridecomposite substrate 1 before, in, or after any one of theabove-described steps. Through the step of adjusting the RMS ofsupport-substrate 11-side main surface 11 n of group III nitridecomposite substrate 1, support-substrate 11-side main surface 11 n canhave a mean value ms of the RMS of 0.3 nm or more and 20 nm or less, andsupport-substrate 11-side main surface 111 n can have a ratios_(S)/m_(S) of standard deviation s_(S) of the RMS to mean value ms ofthe RMS of 0.005 or more and 0.4 or less.

Mean value ms of the RMS of support-substrate 11-side main surface 1 incan be controlled by means of the viscosity of the abrasive, the size ofabrasive grains, the material and the surface shape of a surface plateand a polishing pad, and the polishing conditions. In order to makesmaller mean value ms of the RMS, oil-base abrasives are more preferredrelative to water-base abrasives, a higher viscosity of the abrasive ismore preferred, a smaller abrasive grain size is more preferred, and asoft surface plate and a soft polishing pad are more preferred. As tothe surface shape of the surface plate and the polishing pad, a shapehaving grooves formed therein for sludge removal is preferred. Thegrooves for sludge removal refer to grooves having a relatively widewidth and a relatively wide pitch that are formed for removing awaysludge and/or aggregated abrasive grains on the polishing interface. Thepolishing conditions are preferably a low pressure and a lowcircumferential velocity.

In order to control the ratio s_(S)/m_(S) of standard deviation s_(S) ofthe RMS to mean value ms of the RMS of support-substrate 11-side mainsurface 11 n, a viscosity η (unit: mPa·S) and a flow rate Q (unit: m³/s)of a polishing solution, and an area S (unit: m²), a polishing pressureP (unit: kPa), and a circumferential velocity V (unit: m/s) of a surfaceplate are used so that an action coefficient FE (unit: m²/s) defined bya formula: FE=η×Q×V/S×P is in a predetermined range, specifically4×10⁻¹⁷ or more and 1×10⁻⁶ or less. As to the surface shape of thesurface plate and the polishing pad, a shape having grooves formed formaking the abrasive uniform is preferred. The grooves for making theabrasive uniform refer to grooves having a relatively narrow width and arelatively narrow pitch that are formed for uniformly holding theabrasive in a central portion of the substrate.

<Step of Adjusting RMS of Group III-nitride-Film-Side Main Surface ofGroup III Nitride Composite Substrate>

The method for manufacturing group III nitride composite substrate 1 inthe present reference embodiment includes the step of adjusting the RMSof group III-nitride-film 13-side main surface 13 m by polishing groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1 after the step of forming the group III nitridecomposite substrate. Through the step of adjusting the RMS of groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1, group III-nitride-film 13-side main surface 13 mcan have a mean value m_(III-N) of the RMS of 0.15 nm or more and 3 nmor less, and group III-nitride-film 13-side main surface 13 m can have aratio s_(III-N)/m_(III-N) of standard deviation s_(III-N) of the RMS tomean value m_(III-N) of the RMS of 0.008 or more and 0.5 or less.

Here, mean value m_(III-N) of the RMS of group III-nitride-film 13-sidemain surface 13 m can be controlled by means of the viscosity of theabrasive, the size of abrasive grains, the material and the surfaceshape of a surface plate and a polishing pad, and the polishingconditions. In order to make smaller mean value m_(III-N) of the RMS, ahigher viscosity of the abrasive is more preferred, a smaller abrasivegrain size is more preferred, and a soft surface plate and a softpolishing pad are more preferred.

As to the surface shape of the surface plate and the polishing pad, ashape having grooves formed therein for sludge removal is preferred. Thepolishing conditions are preferably a low pressure and a lowcircumferential velocity.

For the ratio s_(III-N)/m_(III-N) of standard deviation s_(III-N) of theRMS to mean value m_(III-N) of the RMS of group III-nitride-film 13-sidemain surface 13 m, a viscosity η (unit: mPa·S) and a flow rate Q (unit:m³/s) of a polishing solution, and an area S (unit: m²), a polishingpressure P (unit: kPa), and a circumferential velocity V (unit: m/s) ofa surface plate are used so that an action coefficient FE (unit: m²/s)defined by a formula: FE=η×Q×V/S×P is in a predetermined range,specifically 4×10⁻¹⁴ or more and 1×10⁻¹³ or less. As to the surfaceshape of the surface plate and the polishing pad, a shape having groovesformed for making the abrasive uniform is preferred.

Reference Embodiment I-4-2: Another Method for Manufacturing Group IIINitride Composite Substrate

Referring to FIG. 14, a method for manufacturing a group III nitridecomposite substrate, which is a still further reference embodiment ofReference Invention 11, is a method for manufacturing group III nitridecomposite substrate 1 of the Reference Embodiment II-1, including thesteps of: forming joined substrate 1L having a diameter of 75 mm or moreby bonding support substrate 11 and group III nitride film donorsubstrate 13D to each other (FIG. 14 (A) to (C)); forming group IIInitride composite substrate 1 by performing at least one of grinding,polishing, and etching on a main surface of group III nitride film donorsubstrate 13D of joined substrate 1L, the main surface being locatedopposite to the bonded main surface of group III nitride film donorsubstrate 13D (FIG. 14 (D)); and adjusting the RMS of thesupport-substrate-side main surface by polishing thesupport-substrate-side main surface of the group III nitride compositesubstrate before, in, or after any one of the above-described steps.

The method for manufacturing group III nitride composite substrate 1 inthe present reference embodiment can efficiently manufacture low-costand large-diameter group III nitride composite substrate 1 having athick group III nitride film and a small temperature variation on themain surface when a group III nitride layer is grown, to enable groupIII nitride semiconductor devices to be manufactured with a high yield.

Regarding the method for manufacturing group III nitride compositesubstrate 1 in the present reference embodiment, at least one ofgrinding, polishing, and etching is performed on the main surfaceopposite to the bonded main surface of group III nitride film donorsubstrate 13D to thereby adjust the thickness by reducing it, andaccordingly, group III nitride composite substrate 1 including group IIInitride film 13 having a desired thickness of 10 μm or more and 250 μmless can be obtained.

According to the method for manufacturing group III nitride compositesubstrate 1 in the present reference embodiment, in the step of forminga group III nitride composite substrate, at least one of grinding,polishing, and etching is performed on the main surface opposite to thebonded main surface of the group III nitride film donor substrate.Therefore, in order to reduce the material loss of group III nitridefilm donor substrate 13D, the thickness of group III nitride film donorsubstrate 13D to be used is preferably 500 μm or less, and morepreferably 400 μm or less.

<Step of Forming Joined Substrate>

Referring to FIG. 14 (A) to (C), the step of forming joined substrate 1Lincludes, similarly to the method for manufacturing a group III nitridecomposite substrate in Reference Embodiment II-4-1, the sub step offorming joint film 12 a on main surface 11 m of support substrate 11(FIG. 14 (A)), the sub step of forming joint film 12 b on main surface13 n of group III nitride film donor substrate 13D (FIG. 14 (B)), andthe sub step of bonding main surface 12 am of joint film 12 a formed onmain surface 11 m of support substrate 11 and main surface 12 bn ofjoint film 12 b formed on main surface 13 n of group III nitride filmdonor substrate 13D to each other (FIG. 14 (C)).

Here, the sub step of forming joint film 12 a on main surface 11 m ofsupport substrate 11 as shown in FIG. 14 (A) is similar to the sub stepof forming joint film 12 a on main surface 11 m of support substrate 11as shown in FIG. 12 (A). The sub step of forming joint film 12 b on mainsurface 13 n of group III nitride film donor substrate 13D as shown inFIG. 14 (B) is similar to the sub step of forming joint film 12 b onmain surface 13 n of group III nitride film donor substrate 13D as shownin FIG. 12 (B). The sub step of bonding main surface 12 am of joint film12 a formed on main surface 11 m of support substrate 11 and mainsurface 12 bn of joint film 12 b formed on main surface 13 n of groupIII nitride film donor substrate 13D to each other as shown in FIG. 14(C) is similar to the sub step of bonding main surface 12 am of jointfilm 12 a formed on main surface 11 m of support substrate 11 and mainsurface 12 bn of joint film 12 b formed on main surface 13 n of groupIII nitride film donor substrate 13D to each other as shown in FIG. 12(C). Therefore, the description of them will not be repeated.

<Step of Forming Group III Nitride Composite Substrate>

Referring to FIG. 14 (D), in the step of forming group III nitridecomposite substrate 1, at least one of grinding, polishing, and etchingis performed on main surface 13 m opposite to bonded main surface 13 nof group III nitride film donor substrate 13D in joined substrate 1L, tothereby form group III nitride film 13 with its thickness reduced, fromgroup III nitride film donor substrate 13D, and accordingly form groupIII nitride composite substrate 1 in which support substrate 11 andgroup III nitride film 13 are bonded together with joint film 12interposed therebetween.

The method for grinding group III nitride film donor substrate 13D isnot particularly limited, and may be grinding by means of one of agrinding wheel and abrasive grains. The method for polishing group IIInitride film donor substrate 13D is not particularly limited, and may berough polishing such as mechanical polishing, precision polishing suchas CMP and chemical polishing, or the like. The method for etching groupIII nitride film donor substrate 13D is not particularly limited, andmay be wet etching using a chemical solution, dry etching such as RIE(Reactive Ion Etching), or the like.

In order to reduce the thickness variation of group III nitride film 13to be formed, it is preferable to polish main surface 13 m of group IIInitride film 13 of group III nitride composite substrate 1 obtainedthrough at least one of grinding and etching. The reduction of thethickness by at least one of grinding, polishing, and etching ispreferably 10 μm or more, more preferably 20 μm or more, and still morepreferably 30 μm or more, in order to reduce the thickness variation andremove a damaged layer. Meanwhile, the reduction of the thickness by atleast one of grinding, polishing, and etching is preferably 100 μm orless, more preferably 80 μm or less, and still more preferably 60 μm orless, in order to reduce the material loss of group III nitride filmdonor substrate 13D.

<Step of Adjusting RMS of Support-Substrate-Side Main Surface of GroupIII Nitride Composite Substrate>

The method for manufacturing group III nitride composite substrate 1 inthe present reference embodiment includes the step of adjusting the RMSof support-substrate 11-side main surface 11 n by polishingsupport-substrate 11-side main surface 11 n of group III nitridecomposite substrate 1 before, in, or after any one of theabove-described steps. Through the step of adjusting the RMS ofsupport-substrate 11-side main surface 11 n of group III nitridecomposite substrate 1, support-substrate 11-side main surface 11 n canhave a mean value ms of the RMS of 0.3 nm or more and 20 nm or less, andsupport-substrate 11-side main surface 11 n can have a ratio s_(S)/m_(S)of standard deviation s_(S) of the RMS to mean value ms of the RMS of0.005 or more and 0.4 or less.

Here, the control of mean value ms of the RMS of support-substrate11-side main surface 111 n and ratio s_(S)/m_(S) of standard deviations_(S) of the RMS to mean value ms of the RMS of support-substrate11-side main surface 11 n is similar to that of the method formanufacturing group III nitride composite substrate 1 in ReferenceEmbodiment II-4-1.

<Step of Adjusting RMS of Group III-Nitride-Film-Side Main Surface ofGroup III Nitride Composite Substrate>

The method for manufacturing group III nitride composite substrate 1 inthe present reference embodiment preferably includes the step ofadjusting the RMS of group III-nitride-film 13-side main surface 13 m bypolishing group III-nitride-film 13-side main surface 13 m of group IIInitride composite substrate 1 after the step of forming the group IIInitride composite substrate. Through the step of adjusting the RMS ofgroup III-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1, group III-nitride-film 13-side main surface 13 mcan have a mean value m_(III-N) of the RMS of 0.15 nm or more and 3 nmor less, and group III-nitride-film 13-side main surface 13 m can have aratio s_(III-N)/m_(III-N) of standard deviation s_(III-N) of the RMS tomean value mm-N of the RMS of 0.008 or more and 0.5 or less.

Here, the control of mean value m_(III-N) of the RMS of groupIII-nitride-film 13-side main surface 13 m and ratio s_(III-N)/m_(III-N)of standard deviation s_(III-N) of the RMS to mean value m_(III-N) ofthe RMS of group III-nitride-film 13-side main surface 13 m is similarto that of the method for manufacturing group III nitride compositesubstrate 1 in Reference Embodiment II-4-1.

Reference Embodiment II-5: Method for Manufacturing Group III NitrideSemiconductor Device

Referring to FIG. 15, a method for manufacturing a group III nitridesemiconductor device, which is a still further reference embodiment ofReference Invention II, includes the steps of: preparing group IIInitride composite substrate 1 of Reference Embodiment II-1; and growingat least one group III nitride layer 20 on group III-nitride-film13-side main surface 13 m of group III nitride composite substrate 1(FIG. 15 (A)). The method for manufacturing a group III nitridesemiconductor device in the present reference embodiment can be used tomanufacture group III nitride semiconductor devices having excellentcharacteristics with a high yield, since the group III nitride layer isgrown on group III-nitride-film 13-side main surface 13 m having a smalltemperature variation on main surface 13 m when the group III nitridelayer is grown.

The method for manufacturing a group III nitride semiconductor device inthe present reference embodiment may further include the step ofremoving support substrate 11 from group III nitride composite substrate1 (FIG. 15 (C)) after the step of growing group III nitride layer 20(FIG. 15 (A)). This step enables group III nitride semiconductor devicesof a variety of forms to be manufactured.

Further, the method for manufacturing a group III nitride semiconductordevice in the present reference embodiment may further include the stepof bonding device support substrate 40 onto group III nitride layer 20(FIG. 15 (B)) after the step of growing the group III nitride layer(FIG. 15 (A)) and before the step of removing the support substrate(FIG. 15 (C)). This step enables group III nitride semiconductor deviceshaving a high mechanical strength and excellent characteristics andsupported by device support substrate 40 to be manufactured with a highyield.

The method for manufacturing a group III nitride semiconductor device inthe present reference embodiment may specifically be performed throughthe following steps.

<Step of Growing Group III Nitride Layer>

Referring to FIG. 15 (A), in the step of growing at least one group IIInitride layer 20 on group III-nitride-film 13-side main surface 13 m ofgroup III nitride composite substrate 1, suitable methods for growinggroup III nitride layer 20 are gas phase methods such as MOVPE, MBE,HVPE, and sublimation method, as well as liquid phase methods such asflux method, in order to epitaxially grow group III nitride layer 20having a high crystal quality, and a particularly suitable method isMOVPE.

The structure of group III nitride layer 20 varies depending on the typeof group III nitride semiconductor device 4. In the case where group IIInitride semiconductor device 4 is a light-emitting device, group IIInitride layer 20 may be configured by successively growing, on group IIInitride film 13, for example, an n-GaN layer 21, ann-In_(0.05)Ga_(0.95)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25.

In this way, at least one group III nitride layer 20 is grown on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1, and accordingly laminated group III nitridecomposite substrate 2 is obtained.

<Step of Bonding Device Support Substrate>

Referring to FIG. 15 (B), the step of bonding device support substrate40 onto group III nitride layer 20 is performed by forming a firstelectrode 30 and a pad electrode 33 on group III nitride layer 20 oflaminated group III nitride composite substrate 2, forming a padelectrode 43 and a joint metal film 44 on device support substrate 40,and bonding joint metal film 44 to pad electrode 33. Through thesesteps, laminated substrate 3 is obtained. As device support substrate40, Si substrate, CuW substrate, Mo substrate, or the like is used.

<Step of Removing Support Substrate>

Referring to FIG. 15 (C), the step of removing support substrate 11 fromgroup III nitride composite substrate 1 is performed by removing supportsubstrate 11 of group III nitride composite substrate 1 from laminatedsubstrate 3. In the case where group III nitride composite substrate 1includes joint film 12 interposed between support substrate 11 and groupIII nitride film 13, joint film 12 can also be removed.

The method for removing support substrate 11 and joint film 12 is notparticularly limited, and suitable methods to be used are grinding,etching, and the like. For example, support substrate 11 formed of amaterial which is low in hardness, strength, and wear resistance, and islikely to be ground off, can be removed by at least one of grinding andpolishing, in order to reduce the manufacturing cost. Support substrate11 formed of a material which can be dissolved in a chemical solutionsuch as acid solution or alkali solution, can be removed through etchingwith a chemical solution, since it requires low manufacturing cost. Assupport substrate 11, a support substrate formed of a polycrystallinematerial such as ceramic material is more preferred as compared with asupport substrate formed of a monocrystalline material such as sapphire,SiC, group III nitride (GaN for example), or the like, since the supportsubstrate 11 of the polycrystalline material is easier to remove.

It should be noted that, regarding manufacturing of a group III nitridesemiconductor device using a free-standing group III nitride substrate,a rear surface of the free-standing group III nitride substrate (therear surface refers to a main surface opposite to a main surface onwhich a group III nitride layer is formed, and this definition isapplied as well hereinafter) is subjected to grinding or the like forthe purpose of reducing the thickness of the device. In contrast,regarding manufacturing of a group III nitride semiconductor deviceusing a group III nitride composite substrate, it is easy to remove thesupport substrate by etching, grinding, or the like for the purpose ofreducing the thickness of the device, and therefore, the cost ofmanufacturing the group III nitride semiconductor device can be reduced.

<Step of Forming Electrode>

Referring to FIG. 15 (D), on group III nitride film 13 which has beenexposed after removal of support substrate 11 and joint film 12 fromlaminated substrate 3, a second electrode 50 is formed and, on devicesupport substrate 40, a device support substrate electrode 45 is formed.

<<Reference Invention III>>

Reference Invention III relates to a group III nitride compositesubstrate that can be manufactured at a low cost, has a large diameter,and has a group III nitride film with an appropriate thickness and ahigh crystal quality, a method for manufacturing the group III nitridecomposite substrate, a laminated group III nitride composite substrate,as well as a group III nitride semiconductor device and a method formanufacturing the same.

[Background Art Regarding Reference Invention III]

Group III nitrides such as GaN have superior semiconductor propertiesand are therefore used as materials suitable for semiconductor devices.

For example, Japanese Patent Laying-Open No. 2009-126722 discloses afree-standing group III nitride substrate to be used as a substrate fora semiconductor device.

The free-standing group III nitride substrate has a diameter of 25 mm ormore and 160 mm or less and a thickness of 100 μm or more and 1000 μm orless. It discloses, as a specific example thereof, a free-standing GaNsubstrate having a diameter of 10 mm and a thickness of 400 μm.

Japanese Patent Laying-Open No. 2008-010766 discloses aGaN-thin-film-bonded substrate to be used as a substrate formanufacturing a semiconductor device.

The GaN-thin-film-bonded substrate includes a heterogeneous substratewhose chemical composition is different from that of GaN, and a GaN thinfilm having a thickness of 0.1 μm or more and 100 μm or less and bondedto the heterogeneous substrate. It discloses, as a specific examplethereof, a GaN-thin-film-bonded substrate having a diameter of 50.8 mmand including a sapphire substrate and a GaN thin film having athickness of 0.1 μm or 100 μm and bonded to the sapphire substrate.

Problems to be Solved by Reference Invention III

The free-standing group III nitride substrate disclosed in JapanesePatent Laying-Open No. 2009-126722, however, is very expensive. This ischiefly for the reason that the group III nitride does not form a liquidphase and therefore a liquid-phase growth method which is a low-costmanufacturing method cannot be employed, namely it is inevitable toemploy a vapor-phase growth method of a low yield. In addition, thegroup III nitride is a low-fracture-toughness material which is highlysusceptible to fracture. Due to this, it is difficult to increase thediameter of the substrate. Further, if the thickness of the substrate isreduced to a thickness on the order of 250 μm for the purpose ofreducing the manufacturing cost, the substrate is susceptible to warp.Accordingly, in the step of growing an epitaxial layer on the substrate,the crystal quality may be deteriorated or substrate peeling may occur,which reduces the yield of manufacture of semiconductor devices. It hastherefore been difficult to produce a high-value-added semiconductordevice.

In contrast, for the bonded substrate as disclosed in Japanese PatentLaying-Open No. 2008-010766, the amount of use of the expensive groupIII nitride can be reduced, and therefore, the cost of manufacturing asemiconductor device can be reduced.

In order to obtain excellent characteristics of a semiconductor device,it is preferable that the thickness of a group III nitride film which isto serve as a base is preferably 10 μm or more. In the conventionalbonded substrate as disclosed in Japanese Patent Laying-Open No.2008-010766, however, the thickness of the group III nitride film islimited to approximately 1 μm, for the following reasons. For the bondedsubstrate as disclosed in Japanese Patent Laying-Open No. 2008-010766,the heterogeneous substrate and the GaN bulk crystal are bondedtogether, and thereafter the ion implantation method is used to dividethe GaN bulk crystal to thereby form a GaN thin film. On this method,however, a certain limitation in terms of the depth of ion implantationis imposed. Therefore, if a group III nitride film of a 10 μm or more isproduced, the depth to which ions are implanted varies, and consequentlythe thickness of the group III nitride film varies. In this case, anadequate quality cannot be ensured. Namely, for the conventional bondedsubstrate as disclosed in Japanese Patent Laying-Open No. 2008-010766,adequate characteristics of semiconductor devices and an adequate yieldof manufacture of semiconductor devices cannot be achieved if thethickness of the group III nitride film is 10 μm or more.

In the case where a wire saw for example is used instead of the ionimplantation method for the conventional bonded substrate as disclosedin Japanese Patent Laying-Open No. 2008-010766 to cut and separate thegroup III nitride film and thereby obtain the bonded substrate having agroup III nitride film of 10 μm or more, the substrate is susceptible towarp or cracks. In this case as well, it is difficult to grow anepitaxial layer of a high quality on the group III nitride film.

As seen from the above, it has been considerably difficult for theconventional known technique to obtain a group III nitride compositesubstrate which includes a group III nitride film having an appropriatethickness range, and enables excellent semiconductor devicecharacteristics to be achieved, while the amount of use of the expensivegroup III nitride is reduced.

An object of the present invention is to provide a group III nitridecomposite substrate that can be manufactured at a low cost, and has alarge diameter and has a group III nitride film with an appropriatethickness and a high crystal quality, a method for manufacturing thegroup III nitride composite substrate, a laminated group III nitridecomposite substrate, as well as a group III nitride semiconductor deviceand a method for manufacturing the same.

Solution to the Problems

According to an aspect, Reference Invention III provides a group IIInitride composite substrate with a diameter of 75 mm or more including asupport substrate and a group III nitride film having a thickness of 10μm or more and 250 μm or less that are bonded to each other, the groupIII nitride composite substrate includes a joint film interposed betweenthe support substrate and the group III nitride film and joining thesupport substrate and the group III nitride film to each other, and thejoint film has a thickness variation of 2% or more and 40% or less.

According to another aspect, Reference Invention III provides a groupIII nitride composite substrate with a diameter of 75 mm or moreincluding a support substrate and a group III nitride film having athickness of 10 μm or more and 250 μm or less that are bonded to eachother, the group III nitride composite substrate includes a joint filminterposed between the support substrate and the group III nitride filmand joining the support substrate and the group III nitride film to eachother, a shear joint strength between the support substrate and thegroup III nitride film is 4 MPa or more and 40 MPa or less, and a ratioof a joint area between the support substrate and the group III nitridefilm is 60% or more and 99% or less.

Regarding the group III nitride composite substrates according to theseaspects of Reference Invention III, a ratio α_(III-N)/α_(S) of a thermalexpansion coefficient α_(III-N) of the group III nitride film to athermal expansion coefficient α_(S) of the support substrate may be 0.75or more and 1.25 or less, and a ratio t_(III-N)/t_(S) of a thicknesst_(III-N) of the group III nitride film to a thickness t_(S) of thesupport substrate may be 0.02 or more and 1 or less.

The support substrate may have a thermal conductivity λ_(S) of 3W·m⁻¹·K⁻¹ or more and 280 W·m⁻¹·K⁻¹ or less.

The support substrate may have a Young's modulus E_(S) of 150 GPa ormore and 500 GPa or less.

Further, the diameter of the group III nitride composite substrate maybe 125 mm or more and 300 mm or less.

According to still another aspect, Reference Invention III provides alaminated group III nitride composite substrate including the group IIInitride composite substrate according to the above aspect, and at leastone group III nitride layer disposed on the group III-nitride-film-sidemain surface of the group III nitride composite substrate.

According to a further aspect, Reference Invention III provides a groupIII nitride semiconductor device including the group III nitride film inthe group III nitride composite substrate according to the above aspect,and at least one group III nitride layer disposed on the group IIInitride film.

According to a still further aspect, Reference Invention III provides amethod for manufacturing a group III nitride composite substrateaccording to the above aspect, including the steps of: forming a joinedsubstrate with a diameter of 75 mm or more by bonding a supportsubstrate and a group III nitride film donor substrate to each other;and forming the group III nitride composite substrate by cutting thegroup III nitride film donor substrate in the joined substrate along aplane located inwardly at a predetermined distance from a bonded mainsurface of the group III nitride film donor substrate.

According to a still further aspect, Reference Invention III provides amethod for manufacturing a group III nitride composite substrateaccording to the above aspect, including the steps of: forming a joinedsubstrate with a diameter of 75 mm or more by bonding a supportsubstrate and a group III nitride film donor substrate to each other;and forming the group III nitride composite substrate by performing atleast one of grinding, polishing, and etching on a main surface of thegroup III nitride film donor substrate in the joined substrate, the mainsurface being opposite to a bonded main surface of the group III nitridefilm donor substrate.

According to a still further aspect, Reference Invention III provides amethod for manufacturing a group III nitride semiconductor device,including the steps of: preparing a group III nitride compositesubstrate; and growing at least one group III nitride layer on a groupIII nitride film of the group III nitride composite substrate.

The method for manufacturing a group III nitride semiconductor deviceaccording to this aspect of Reference Invention III may further includethe steps of: bonding a device support substrate onto the group IIInitride layer; and removing the support substrate from the group IIInitride composite substrate.

[Effects of Reference Invention III]

Reference Invention III can provide a group III nitride compositesubstrate that can be manufactured at a low cost, and has a largediameter and has a group III nitride film with an appropriate thicknessand a high crystal quality, a method for manufacturing the group IIInitride composite substrate, a laminated group III nitride compositesubstrate, as well as a group III nitride semiconductor device and amethod for manufacturing the same.

Reference Embodiment III-1: Group III Nitride Composite Substrate

Referring to FIG. 6, a group III nitride composite substrate 1 which isa reference embodiment of Reference Invention III is a substrate with adiameter of 75 mm or more including a support substrate 11 and a groupIII nitride film 13 having a thickness of 10 μm or more and 250 μm orless that are bonded to each other. Group III nitride compositesubstrate 1 includes a joint film 12 interposed between supportsubstrate 11 and group III nitride film 13 and joining support substrate11 and group III nitride film 13 to each other. The group III nitridecomposite substrate is characterized in that joint film 12 has athickness variation of 2% or more and 40% or less.

Group III nitride composite substrate 1 in the present referenceembodiment is also characterized in that a shear joint strength betweensupport substrate 1 and group III nitride film 13 joined to each otherby joint film 12 is 4 MPa or more and 40 MPa or less, and a ratio of ajoint area between support substrate 11 and group III nitride film 13 is60% or more and 99% or less.

In contrast to the conventional free-standing group III nitridesubstrate, the present reference embodiment provides a compositesubstrate in which group III nitride film 13 is joined to supportsubstrate 11. This feature can be employed to reduce the thickness ofthe expensive group III nitride film and reduce the cost of thesemiconductor device.

Further, also in contrast to the conventional bonded substrate, thepresent reference embodiment provides the substrate having a largediameter of 75 mm or more and group III nitride film 13 having athickness of 10 μm or more and 250 μm or less.

Accordingly, the group III nitride composite substrate of the presentreference embodiment can be used to obtain significantly excellentsemiconductor device characteristics.

The above-described features can be obtained from that the thicknessvariation of joint film 12 is 2% or more and 40% or less, or that theshear joint strength between support substrate 11 and group III nitridefilm 13 is 4 MPa or more and 40 MPa or less, and the ratio of the jointarea between support substrate 11 and group III nitride film 13 is 60%or more and 99% or less.

The group III nitride composite substrate includes joint film 12 havingits thickness variation which is controlled so that the thicknessvariation falls within a specific range. Therefore, even in the casewhere group III nitride film 13 of a substrate having a large diameterof 75 mm or more is cut with a wire saw or the like to separate a partthereof and thereby reduce the thickness thereof to 10 μm or more and250 μm or less, the heat from a susceptor on which the substrate ismounted is transmitted uniformly in the film in a semiconductor deviceprocess of forming an epitaxial layer on group III nitride film 13.Thus, the epitaxial layer having a good thickness variation and a highcrystal quality can be obtained, and accordingly, the yield ofmanufacture of semiconductor devices can also be increased.

The joint strength and the ratio of the joint area between supportsubstrate 11 and group III nitride film 13 are controlled so that theyfall within specific ranges respectively. Therefore, the stress appliedto the joint film is alleviated and occurrence of warp can besuppressed. Accordingly, the yield of manufacture of semiconductordevices can be increased.

Group III nitride composite substrate 1 in the present referenceembodiment can have at least one of: the above-described feature of thethickness variation of joint film 12; and the above-described feature ofthe joint strength and the ratio of the joint area between supportsubstrate 11 and group III nitride film 13, to thereby increase theyield of manufacture of semiconductor devices and improve thecharacteristics of the semiconductor devices. It is particularlypreferable that group III nitride composite substrate 1 has both theabove-described features, since respective effects of the features aresynergistically produced and thus the effects of the present inventioncan further be enhanced.

<Diameter of Group III Nitride Composite Substrate>

In order to produce a greater number of semiconductor device chips fromone composite substrate, the diameter of group III nitride compositesubstrate 1 is 75 mm or more, and is preferably 100 mm or more, morepreferably 125 mm or more, and still more preferably 150 mm or more.Meanwhile, in order to reduce the warp of the composite substrate andincrease the yield of semiconductor devices, the diameter of group IIInitride composite substrate 1 is preferably 300 mm or less, and morepreferably 200 mm or less.

In the following, each of the parts constituting group III nitridecomposite substrate 1 in the present reference embodiment will bedescribed.

<Joint Film>

Joint film 12 in the present reference embodiment has a function ofabsorbing and alleviating unevenness of a joint surface of supportsubstrate 11 and a joint surface of group III nitride film 13 to therebyincrease the joint strength between support substrate 11 and group IIInitride film 13.

Joint film 12 is not particularly limited as long as joint film 12 canjoin support substrate 11 and group III nitride film 13 to each other.Joint film 12 is preferably SiO₂ film, Si₃N₄ film, TiO₂ film, Ga₂O₃film, or the like, because these films have a high joining ability forjoining support substrate 11 and group III nitride film 13 to eachother. While the average thickness of joint film 12 is not particularlylimited, the average thickness thereof may for example be approximately100 nm to 4 μm.

<Thickness Variation of Joint Film>

In the present reference embodiment, the thickness variation of jointfilm 12 is 2% or more and 40% or less. “Thickness variation” is hereinan indicator of the uniformity of the thickness of joint film 12 andcalculated in accordance with the following formula, from “maximum valuet_(max) of the thickness” and “minimum value t, of the thickness” amongthe thicknesses measured in the entire main surface of joint film 12.

thickness variation (%)={(t _(max) −t _(min))/(t _(max) +t_(min))}×100  Formula:

Here, as a reference surface for the thickness of the joint film, mainsurface 11 m of support substrate 11 may be used. The measurement pointswhere the thickness is measured are preferably at least 13 points, andthe measurement points adjacent to each other are preferably located atsubstantially regular intervals.

The thickness of the joint film can be measured with a conventionallyknown optical-interference film thickness meter, step meter, or thelike. The thickness can also be measured through observation of a crosssection perpendicular to the main surface of joint film 12 with ascanning electron microscope (SEM) or the like.

If the thickness variation is less than 2%, heat conduction from thesusceptor on which the substrate is mounted is nonuniform when anepitaxial layer is grown. As a result, the substrate is warped in aconcave form and has a large difference between the temperature of acentral portion and the temperature of a peripheral portion. In thiscase, the epitaxial layer with a high quality cannot be grown, the yieldof manufacture of semiconductor devices is low, and the characteristicsof the semiconductor devices are deteriorated. If the thicknessvariation is more than 40%, there are a greater number of regions wherethe joint film is thinner and regions where the joint film is absent(namely non-joined regions). In this case as well, an epitaxial layerwith a high quality cannot be grown and the yield of manufacture ofsemiconductor devices is low.

In view of the above, the thickness variation of the joint film in thepresent reference embodiment is 2% or more and 40% or less. Thethickness variation falling within this range produces superior effectsthat the temperature is distributed uniformly over the whole compositesubstrate during epitaxial growth and a high-quality epitaxial layerhaving a high crystal quality can be grown. The thickness variation ismore preferably 5% or more and 25% or less, and still more preferably 7%or more and 16% or less.

The thickness variation falling within these ranges enables theuniformity of the thickness of the joint film to be further improved andthe crystal quality of the epitaxial layer formed on group III nitridefilm 13 to be further enhanced.

The thickness variation of the joint film can be controlled so that itfalls within a desired range, by appropriately adjusting the conditionsof chemical mechanical polishing (hereinafter also referred to as “CMP”)performed on the surface of the joint film, for example. Examples of theconditions may be the material for the abrasive, the linear velocity ofpolishing, the material for the polishing pad, and the like.

<Shear Joint Strength>

In the present reference embodiment, the shear joint strength betweensupport substrate 11 and group III nitride film 13 joined together byjoint film 12 is 4 MPa or more and 40 MPa or less. In the case where theshear joint strength falls in this range, substrate peeling does notoccur and the warp of the substrate is alleviated in a process ofmanufacturing a semiconductor device. Therefore, the yield ofmanufacture of semiconductor devices is remarkably increased. The shearjoint strength is more preferably 10 MPa or more and 30 MPa or less.This is suitable since such a shear strength tends to further increasethe effect of alleviating the warp of the substrate. If the shear jointstrength is less than 4 MPa, the joint strength is not sufficient. Inthis case, substrate peeling occurs during epitaxial growth, because ofdeformation of the substrate due to heat conduction from the susceptoron which the substrate is mounted, resulting in a lower yield ofmanufacture of semiconductor devices. If the shear joint strength ismore than 40 MPa, a greater stress is applied to joint film 12 and thewarp of the substrate tends to be increased, resulting in a lower yieldof manufacture of semiconductor devices.

In the present reference embodiment, the shear joint strength can bemeasured with a die shear tester, tensile tester, or the like, by amethod in accordance with JIS K 6850 “Determination of tensile lap-shearstrength of rigid-to-rigid bonded assemblies.” Specifically, arectangular composite substrate (6 mm in length×8 mm in width) isprepared as a measurement sample, and the composite substrate is placedflat and secured on a sample stage of a tester so that the supportsubstrate faces downward. After this, a test jig of 9 mm in width isused to apply a load in the direction parallel to the joint surfacebetween the support substrate and the group III nitride film (namely theshear direction), and a maximum shear load at the time when the jointsurface is broken is measured. Then, the maximum shear load is dividedby the area of the joint surface (4.8×10⁻⁵ m²), to thereby calculate theshear joint strength.

As a method for achieving a shear joint strength of 4 MPa or more and 40MPa or less between support substrate 11 and group III nitride film 13,a method according to which an annealing treatment is performed beforeand after support substrate 11 and group III nitride film 13 are joinedto each other, for example, can appropriately be used. Namely, asuitable method is as follows. After a joint film is formed on each ofone main surface of support substrate 11 and one main surface of groupIII nitride film 13, each of support substrate 11 and group III nitridefilm 13 is subjected to an annealing treatment. Annealed supportsubstrate 11 and annealed group III nitride film 13 are joined to eachother with the joint film interposed therebetween, and thereaftersubjected again to the annealing treatment.

Regarding conditions for the annealing treatment, the annealingtreatment is performed preferably in a nitrogen atmosphere at 400° C. ormore for one hour or more, more preferably in a nitrogen atmosphere at600° C. or more for one hour or more, and particularly preferably in anitrogen atmosphere at 800° C. or more for one hour or more.

In terms of the quality of the joint film, the temperature condition forthe annealing treatment is preferably 1200° C. or less, and the time forthe treatment is preferably 48 hours or less.

The shear joint strength can also be controlled by means of the surfacestate (namely the surface roughness) before the joint film is joined.

<Ratio of Joint Area>

Group III nitride composite substrate 1 in the present referenceembodiment needs to have a shear joint strength between supportsubstrate 11 and group III nitride film 13 of 4 MPa or more and 40 MPaor less, and a ratio of the joint area between support substrate 11 andgroup III nitride film 13 of 60% or more and 99% or less as describedabove. The relation between support substrate 11 and group III nitridefilm 13 is thus defined in terms of these two respects, and therefore,group III nitride composite substrate 1 in the present referenceembodiment enables a warp of the substrate during epitaxial growth to beremarkably reduced and enables a high-flatness and high-qualityepitaxial layer to be grown. Accordingly, the composite substrate hassuperior effects that the frequency of occurrence of substrate peelingis extremely low in the process of manufacturing a semiconductor deviceand the yield of manufacture of semiconductor devices is high. In thecase where the ratio of the joint area is less than 60%, the frequencyof occurrence of substrate peeling is high in the process of epitaxialgrowth and the process of manufacturing a semiconductor device,resulting in a lower yield of manufacture of semiconductor devices. Inthe case where the ratio of the joint area is higher than 99%, a greaterstress is applied to joint film 12 and a warp is likely to occur to thesubstrate. Therefore, in this case as well, the yield of manufacture ofsemiconductor devices is lower.

In the present reference embodiment, “ratio of joint area” is a valuedetermined by calculating the sum of areas detected as joint defects(void or peeling) through observation of joint film 12 which is a jointsurface between support substrate 11 and group III nitride film 13 withan ultrasonic microscope, dividing the sum by the area of main surface11 m of support substrate 11, and multiplying the quotient by 100. Theratio of the joint area is more preferably 70% or more and 90% or less,and still more preferably 80% or more and 86% or less. In the case wherethe ratio of the joint area falls in these ranges, the stress applied tojoint film 12 is significantly alleviated, and the yield of manufactureof semiconductor devices can further be increased.

As a method for achieving a ratio of the joint area of 60% or more and99% or less, a method according to which the surface of joint film 12 iscleaned, for example, can be used. Specifically, a method canappropriately be used according to which dirt on the surface of jointfilm 12 is removed by CMP and thereafter the surface is furthersubjected to ultrasonic cleaning with water. As a more preferred method,a method can be used according to which dirt on the surface of jointfilm 12 is removed by CMP and thereafter dirt is further removed bynon-abrasive polishing cleaning with a chemical solution such as anaqueous solution of potassium hydroxide (KOH) or water.

Both the ultrasonic cleaning and the non-abrasive polishing cleaning,for example, may also be used.

The ratio of the joint area can more precisely be controlled by thethickness variation of joint film 12 defined as 2% or more and 40% orless. Namely, it is particularly preferable that the thickness variationof joint film 12 is 2% or more and 40% or less and the ratio of thejoint area is 60% or more and 99% or less.

<Support Substrate>

Support substrate 11 is not particularly limited as long as supportsubstrate 11 can support group III nitride film 13. In order to reducethe amount of use of the expensive group III nitride to thereby lowerthe cost, however, the support substrate is preferably ahetero-composition substrate whose chemical composition is differentfrom that of the group III nitride. Support substrate 11 may be eithertransparent or opaque, and one of the transparent one and the opaque onecan appropriately be selected depending on a semiconductor device to beused.

As the material forming support substrate 11, a conventionally knownceramic material, semiconductor material, metal material,polycrystalline material, monocrystalline material, or the like can beused. For example, the material for support substrate 11 may be asintered body material of aluminum nitride (AlN), spinel (MgAl₂O₄),mullite (3Al₂O₃.2SiO₂-2Al₂O₃.SiO₂), alumina (Al₂O₃), graphite, or thelike, a monocrystalline material such as AlN, sapphire, or the like, ametal material such as molybdenum (Mo), tungsten (W), or the like, or analloy material such as copper-tungsten (Cu—W), or the like.

Support substrate 11 may be exposed to a high-temperature corrosive gassuch as ammonia gas during epitaxial growth for example, and istherefore preferably a substrate resistant to corrosion. Thus, thesubstrate may be covered with any of various surface protection coatingsfor improving the corrosion resistance of its surface.

<Thermal Conductivity of Support Substrate>

Support substrate 1I has a thermal conductivity λ_(S) of preferably 3W·m⁻¹·K⁻¹ or more and 280 W·m⁻¹·K⁻¹ or less, more preferably 5 W·m⁻¹·K⁻¹or more and 210 W·m⁻¹·K⁻¹ or less, and still more preferably 10W·m⁻¹·K⁻¹ or more and 120 W·m⁻¹·K⁻¹ or less. Thermal conductivity s ofsupport substrate 11 can be measured by a laser flash method. Group IIInitride composite substrate 1 having support substrate 11 with a thermalconductivity λ_(S) of preferably 3 W·m⁻¹·K⁻¹ or more, more preferably 5W·m⁻¹·K⁻¹ or more, and still more preferably 10 W·m⁻¹·K⁻¹ or more, canefficiently transmit heat from the main surface of the susceptor to mainsurface 13 m of group III nitride film 13 of group III nitride compositesubstrate 1, when a group III nitride layer is grown. Group III nitridecomposite substrate 1 having support substrate 11 with a thermalconductivity λ_(S) of preferably 280 W·m⁻¹·K⁻¹ or less, more preferably210 W·m⁻¹·K⁻¹ or less, and still more preferably 120 W·m⁻¹·K⁻¹ or less,can uniformly transmit heat from the main surface of the susceptor tothe whole main surface of group III nitride film 13 of group III nitridecomposite substrate 1, when a group III nitride layer is grown. Supportsubstrate 11 having a thermal conductivity λ_(S) of 280 W·m⁻¹·K⁻¹ orless can more uniformly transmit the heat from the main surface of thesusceptor to the whole main surface of group III nitride film 13 ofgroup III nitride composite substrate 1 when a group III nitride layeris grown, as compared with the case where an SiC substrate having athermal conductivity λ_(S) of about 300 W·m⁻¹·K⁻¹ is used as the supportsubstrate. It should be noted that the thermal conductivity of supportsubstrate 11 may be different from the thermal conductivity of group IIInitride film 13.

<Thermal Expansion Coefficient of Support Substrate>

Support substrate 11 is preferably a substrate which is not prone tocrack. The thermal expansion coefficient of support substrate 11 ispreferably close to the thermal expansion coefficient of group IIInitride film 13. Support substrate 11 having such properties isappropriate, since it makes group III nitride composite substrate 1 lessprone to crack even when group III nitride composite substrate 1 isheated in the process of epitaxial growth, the process of manufacturinga semiconductor device, or the like.

Specifically, a ratio α_(III-N)/α_(S) of a thermal expansion coefficientα_(III-N) of group III nitride film 13 to a thermal expansioncoefficient α_(S) of support substrate 11 is preferably 0.75 or more and1.25 or less, more preferably 0.8 or more and 1.2 or less, still morepreferably 0.9 or more and 1.1 or less, and particularly preferably 0.95or more and 1.05 or less.

<Thickness of Support Substrate 11>

While the thickness itself of support substrate 11 is not particularlylimited, it is preferable that the thickness of support substrate 11 andthe thickness of group III nitride film 13 satisfy the followingrelation, in order to suppress warp, crack, or the like of group IIInitride film 13 when it is heated. Namely, a ratio t_(III-N)/t_(S) of athickness t_(III-N) of group III nitride film 13 to a thickness t_(S) ofsupport substrate 11 is preferably 0.02 or more and 1 or less. The factthat the ratio α_(III-N)/α_(S) of the thermal expansion coefficient is0.75 or more and 1.25 or less and the ratio t_(III-N)/t_(S) of thethickness is 0.02 or more and 1 or less makes it possible tosignificantly reduce occurrences of defective pieces due to warp orcrack of group III nitride film 13, on various occasions such as aprocess of manufacturing the composite substrate, a process of epitaxialgrowth, and a process of manufacturing a semiconductor device. The ratiot_(III-N)/t_(S) of the thickness is more preferably 0.07 or more and 0.5or less.

<Young's Modulus of Support Substrate>

Regarding Young's modulus E_(S) of support substrate 11, E_(S) ispreferably 150 GPa or more and 500 GPa or less, in order to suppressoccurrence of warp when group III nitride composite substrate 1 isheated. If E_(S) is less than 150 GPa, warp tends to be likely to occurduring heating. If E_(S) is more than 500 GPa, breakage or crack tendsto be likely to occur during heating. These E_(s) are therefore notpreferred. E_(S) is more preferably in a range of 200 GPa or more and350 GPa or less. While the Young's modulus of support substrate 11 maybe different from that of group III nitride film 13, preferably Young'smodulus of support substrate 11 is substantially identical to that ofgroup III nitride film 13.

Regarding the material which forms support substrate 11, supportsubstrate 11 may be formed of a material having its thermal expansioncoefficient and its Young's modulus that are close to those of group IIInitride film 13, such as a substrate formed of mullite(3Al₂O₃.2SiO₂-2Al₂O₃.SiO₂), a substrate formed of mullite-YSZ (YttriaStabilized Zirconia), a substrate formed of spinel (MgAl₂O₄), asubstrate formed of a sintered body of an Al₂O₃—SiO₂-based compositeoxide, and substrates formed respectively of sintered bodies of them towhich oxide, nitride, carbonate or the like is added, a molybdenum (Mo)substrate, a tungsten (W) substrate, and the like. Here, preferredelements to be contained in the oxide, the nitride, and the carbonateare Ca, Mg, Sr, Ba, Al, Sc, Y, Ce, Pr, Si, Ti, Zr, V, Nb, Ta, Cr, Mn,Fe, Co, Ni, Cu, Zn, and the like.

<Group III Nitride Film>

Group III nitride film 13 is a film formed of a group III nitride,specifically an In_(x)Al_(y)Ga_(1-x-y)N film (0≤x, 0≤y, x+y≤1) such asGaN film, AlN film, or the like.

The thickness of group III nitride film 13 is 10 μm or more and 250 μmor less. In the case where the thickness is less than 10 μm, adequatesemiconductor device characteristics tend to fail to be obtained. If thethickness is more than 250 μm, the amount of use of the expensive groupIII nitride is larger, which makes it difficult to produce ahigh-value-added semiconductor device. In order to enhance thesemiconductor device characteristics, the thickness of group III nitridefilm 13 is preferably 30 μm or more, more preferably 80 μm or more, andparticularly preferably 100 μm or more. In order to produce ahigh-value-added semiconductor device, the thickness is preferably 200μm or less, more preferably 180 μm or less, and particularly preferably130 μm or less.

The crystal structure of group III nitride film 13 is preferably thewurtzite structure, since it enables semiconductor devices havingexcellent characteristics to be produced. The predetermined planeorientation to which the main surface of group III nitride film 13 isclosest is not limited as long as it is suitable for a desiredsemiconductor device, and may be any of {0001}, {10-10}, {11-20},{21-30}, {20-21}, {10-11}, {11-22}, and {22-43}, as well as planeorientations that are displaced by an angle of 15° or less (off by 15°or less) from these plane orientations, respectively. It may also be anyof the plane orientations opposite to the above-listed planeorientations, as well as plane orientations that are 15° or less offfrom these plane orientations, respectively. Namely, main surface 13 mof group III nitride film 13 may be any of polar plane, nonpolar plane,and semipolar plane. Main surface 13 m of group III nitride film 13 ispreferably the {0001} plane and the opposite plane thereof, since suchplanes make it easy to increase the diameter, or any of {10-10} plane,{20-21} plane, and the opposite planes thereof, since such planessuppress blue shift of a light-emitting device to be produced.

Impurity metal atoms in main surface 13 m of group III nitride film 13are preferably 3×10¹² atoms/cm² or less, more preferably 1×10¹²atoms/cm² or less, and still more preferably 1×10¹¹ atoms/cm² or less,in order to enhance the crystal quality of a group III nitride layergrown on group III nitride film 13 and enhance the characteristics of asemiconductor device to be formed. Group III nitride composite substrate1 which includes support substrate 11 such as a substrate of mullite(3Al₂O₃.2SiO₂-2Al₂O₃.SiO₂), mullite-YSZ (Yttria Stabilized Zirconia),spinel (MgAl₂O₄), a sintered body of an Al₂O₃—SiO₂-based compositeoxide, or the like is preferably subjected to cleaning that suppresseselution of metal atoms from support substrate 11, such as scrub cleaningwith a surfactant and pure water, two-fluid cleaning, megasoniccleaning, sheet-fed cleaning of a single side with low-concentrationacid or alkali, for example, to thereby reduce the concentration ofimpurity metal atoms in main surface 13 m of group III nitride film 13.

Regarding other impurities in main surface 13 m of group III nitridefilm 13, in order to enhance the crystal quality of a group III nitridelayer grown on group III nitride film 13 and enhance the characteristicsof a semiconductor device to be formed, the impurities are preferably Clatoms of 2×10¹⁴ atoms/cm² or less, Si atoms of 9×10¹³ atoms/cm² or less.The dislocation density of group III nitride film 13 is not particularlylimited. In order to reduce leak current of the semiconductor device,the dislocation density is preferably 1×10⁸ cm² or less. The carrierconcentration of group III nitride film 13 is not particularly limited.In order to reduce the resistance of the semiconductor device, thecarrier concentration is preferably 1×10¹⁷ cm⁻³ or more.

The above-described group III nitride composite substrate can bemanufactured through the following manufacturing method. Namely, thegroup III nitride composite substrate obtained through the followingmanufacturing method can be manufactured at a low cost, has a largediameter, and has a group III nitride film with an appropriate thicknessand a high crystal quality.

Reference Embodiment III-2: Method for Manufacturing Group III NitrideComposite Substrate

Referring now to FIGS. 12 and 14, a method for manufacturing group IIInitride composite substrate 1, which is another reference embodiment ofReference Invention III, is not particularly limited as long as it is amethod according to which group III nitride film 13 is disposed onsupport-substrate 11-side main surface 11 n, and may be any of thefollowing first and second methods.

The first method is a method as shown in FIG. 12 according to which agroup III nitride film donor substrate 13D is bonded to main surface 1 mof support substrate 11, and thereafter group III nitride film donorsubstrate 13D is cut along a plane located at a predetermined depth fromthe bonded surface, to thereby form group III nitride film 13 on mainsurface 11 m of support substrate 11.

The second method is a method as shown in FIG. 14 according to which agroup III nitride film donor substrate 13D is bonded to main surface 11m of support substrate 11, and thereafter at least one of grinding,polishing, and etching is performed on a main surface of group IIInitride film donor substrate 13D, which is a main surface opposite tothe bonded surface thereof, to adjust the thickness by reducing thethickness, and thereby form group III nitride film 13 on main surface 11m of support substrate 1.

Regarding the above-described first and second methods, the method forbonding group III nitride film donor substrate 13D to support substrate11 may be a method according to which group III nitride film donorsubstrate 13D is bonded to main surface 11 m of support substrate 11with a joint film 12 interposed therebetween (see FIGS. 12 and 14), orthe like.

FIGS. 12 and 14 illustrate a method according to which a joint film 12 ais formed on support substrate 11, a joint film 12 b is formed on groupIII nitride film 13, and these joint films are bonded to each other.Alternatively, for example, joint film 12 may be formed on only supportsubstrate 11 and this joint film may be bonded to group III nitride film13.

<First Method: Cutting Method>

The method for manufacturing a composite substrate by the first methodillustrated in FIG. 12 is not particularly limited. In order toefficiently manufacture the composite substrate, the method preferablyincludes the steps of: forming a joined substrate 1L by bonding supportsubstrate 11 and group III nitride film donor substrate 13D to eachother (FIG. 12 (A) to (C)); and cutting group III nitride film donorsubstrate 13D along a plane located inwardly at a predetermined depthfrom main surface 13 n, which is the bonded surface, of group IIInitride film donor substrate 13D of joined substrate 1L (FIG. 12 (D)).

Support substrate 11 is not particularly limited, and can be obtainedfor example by mixing, at a predetermined molar ratio, MO, (x is anarbitrary positive real number) which is an oxide containing a metalelement M, Al₂O₃ which is an oxide containing Al, and SiO₂ which is anoxide containing Si, sintering the resultant mixture, cutting asubstrate of a predetermined size from the resultant sintered body, andpolishing a main surface of this substrate.

Group III nitride film donor substrate 13D is a donor substrate whichprovides group III nitride film 13 by being separated in a later step.As a method for forming such group III nitride film donor substrate 13D,MOCVD (Metal Organic Chemical Vapor Deposition), sputtering, MBE(Molecular Beam Epitaxy), PLD (Pulsed Laser Deposition), HVPE (HydrideVapor Phase Epitaxy), sublimation method, flux method, high nitrogenpressure solution method, or the like can appropriately be used.

As shown in FIG. 12 (A) to (C), the step of forming joined substrate 1Lby bonding support substrate 11 and group III nitride film donorsubstrate 13D to each other includes: the sub step of forming joint film12 a on main surface 11 m of support substrate 11 (FIG. 12 (A)); the substep of forming joint film 12 b on main surface 13 n of group IIInitride film donor substrate 13D (FIG. 12 (B)); and the sub step ofbonding joint film 12 a formed on main surface 11 m of support substrate11 and joint film 12 b formed on main surface 13 n of group III nitridefilm donor substrate 13D to each other (FIG. 12 (C)). These sub stepsare performed to join and thereby integrate joint film 12 a and jointfilm 12 b bonded to each other into joint film 12, and support substrate11 and group III nitride film donor substrate 13D are joined to eachother with joint film 12 interposed therebetween to thereby form joinedsubstrate 1L.

Here, the method for forming joint films 12 a, 12 b is not particularlylimited. In order to reduce the cost of forming the films, sputtering,vapor deposition, CVD (Chemical Vapor Deposition), or the like isappropriately performed. The method for bonding joint film 12 a andjoint film 12 b to each other is not particularly limited, and suitablemethods include: a direct bonding method according to which respectivesurfaces to be bonded to each other are cleaned, directly bondedtogether, and thereafter heated to approximately 600° C. to 1200° C. sothat the joint films are joined together; a surface activated bondingmethod according to which the surfaces to be bonded to each other arecleaned, subjected to an activation treatment by means of plasma orions, and thereafter joined together under a low-temperature atmosphereof room temperature (25° C. for example) to 400° C.; a high-pressurebonding method according to which the surfaces to be bonded to eachother are cleaned with a chemical solution and pure water and thereaftersubjected to a high pressure on the order of 0.1 MPa to 10 MPa so thatthe joint films are joined together; a high vacuum bonding methodaccording to which the surfaces to be bonded to each other are cleanedwith a chemical solution and pure water, and thereafter joined togetherunder a high vacuum atmosphere on the order of 10⁻⁶ Pa to 10⁻³ Pa, andthe like. Any of the above-referenced bonding methods can increase thetemperature to approximately 600° C. to 1200° C. after the joint filmsare joined together to thereby further increase the joint strength. Inparticular, the surface activated bonding method, the high pressurebonding method, and the high vacuum bonding method exhibit a highereffect of increasing the joint strength through the heating toapproximately 600° C. to 1200° C. after the joint films are joinedtogether.

The step of cutting along a plane located inwardly at a predetermineddepth from main surface 13 n, which is the bonded surface, of group IIInitride film donor substrate 13D of joined substrate 1L shown in FIG. 12(D) is performed by cutting group III nitride film donor substrate 13Dalong a plane located inwardly at a predetermined depth from mainsurface 13 n, which is the bonded surface, of group III nitride filmdonor substrate 13D of joined substrate 1L. The method for cutting groupIII nitride film donor substrate 13D is not particularly limited, andwire saw, inner circumferential blade, outer circumferential blade, orthe like is appropriately used.

In this way, joined substrate 1L is cut along a plane located inwardlyat a predetermined depth from main surface 13 n which is the bondedsurface of group III nitride film donor substrate 13D, and accordinglygroup III nitride composite substrate 1 including support substrate 11,joint film 12 disposed on main surface 11 m of support substrate 11, andgroup III nitride film 13 disposed on the main surface of joint film 12is obtained.

<Second Method: Grinding, Polishing, and Etching Methods>

The method for manufacturing a composite substrate by the second methodas illustrated in FIG. 14 is not particularly limited. In order toefficiently manufacture the composite substrate, the method preferablyincludes the steps of: forming joined substrate 1L by bonding supportsubstrate 11 and group III nitride film donor substrate 13D to eachother (FIG. 14 (A) to (C)); and performing at least one of grinding,polishing, and etching on main surface 13 m located opposite to mainsurface 13 n which is the bonded surface of group III nitride film donorsubstrate 13D of joined substrate 1L (FIG. 14 (D)).

As shown in FIG. 14 (A) to (C), the step of forming joined substrate 1Lby bonding support substrate 11 and group III nitride film donorsubstrate 13D to each other includes: the sub step of forming joint film12 a on main surface 11 m of support substrate 11 (FIG. 14 (A)); the substep of forming joint film 12 b on main surface 13 n of group IIInitride film donor substrate 13D (FIG. 14 (B)); and the sub step ofbonding joint film 12 a formed on main surface 11 m of support substrate11 and joint film 12 b formed on main surface 13 n of group III nitridefilm donor substrate 13D to each other (FIG. 14 (C)). These sub stepsare performed to join and thereby integrate joint film 12 a and jointfilm 12 b bonded to each other into joint film 12, and support substrate11 and group III nitride film donor substrate 13D are joined to eachother with joint film 12 interposed therebetween to thereby form joinedsubstrate 1L.

The method for forming group III nitride film donor substrate 13D issimilar to the method for forming group III nitride film donor substrate13D according to the first method as described above. The method forforming joint films 12 a, 12 b is also similar to the method for formingjoint films 12 a, 12 b in the method for manufacturing a compositesubstrate according to the first method. The method for bonding supportsubstrate 11 and group III nitride film donor substrate 13D to eachother is also similar to the method for bonding support substrate 11 andgroup III nitride film 13 to each other in the method for manufacturinga composite substrate according to the first method as described above.

As shown in FIG. 14 (D), by the step of performing at least one ofgrinding, polishing, and etching on main surface 13 m opposite to mainsurface 13 n which is the bonded surface of group III nitride film donorsubstrate 13D of joined substrate 1L, the thickness of group III nitridefilm donor substrate 13D is reduced to thereby form group III nitridefilm 13 having a desired thickness. Accordingly, group III nitridecomposite substrate 1 including support substrate 11, joint film 12disposed on main surface 11 m of support substrate 11, and group IIInitride film 13 disposed on the main surface of joint film 12 isobtained.

Here, the method for grinding group III nitride film donor substrate 13Dis not particularly limited, and may be grinding with a grinding wheel(surface grinding), shot blast, or the like. The method for polishinggroup III nitride film donor substrate 13D is not particularly limited,and may be mechanical polishing, chemical mechanical polishing, or thelike. The method for etching group III nitride film donor substrate 13Dis not particularly limited, and may be wet etching with a chemicalsolution, dry etching such as RIE (Reaction Ion Etching), or the like.

In this way, group III nitride composite substrate 1 can bemanufactured.

Group III nitride composite substrate 1 manufactured in theabove-described way has excellent effects that a high-quality epitaxiallayer can be grown on the composite substrate and the yield ofmanufacture of semiconductor devices is improved.

Reference Embodiment III-3: Laminated Group III Nitride CompositeSubstrate

Referring now to FIG. 8, a laminated group III nitride compositesubstrate 2 which is still another embodiment of Reference Invention IIIwill be described.

Laminated group III nitride composite substrate 2 includes group IIInitride composite substrate 1 of Reference Embodiment III-1, and atleast one group III nitride layer 20 disposed on group III-nitride-film13-side main surface 13 m of group III nitride composite substrate 1.

Thus, group III nitride layer 20 is disposed on group III-nitride-film13-side main surface 13 m of group III nitride composite substrate 1 andtherefore, group III nitride layer 20 can be grown as a high-qualityepitaxial layer.

In laminated group III nitride composite substrate 2 of the presentreference embodiment, group III nitride layer 20 disposed on groupIII-nitride-film 13-side main surface 13 m varies depending on the typeof the semiconductor device to be produced. Referring to FIG. 9, in thecase where the semiconductor device to be produced is a light-emittingdevice, group III nitride layer 20 may be configured to include, forexample, an n-GaN layer 21, an n-In_(0.05)Ga_(0.95)N layer 22, an activelayer 23 having a multiple quantum well structure, ap-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaN layer 25. Referring to FIG.10, in the case where the semiconductor device to be produced is an HEMT(High Electron Mobility Transistor) which is an example of electronicdevices, the group III nitride layer may be configured to include, forexample, a GaN layer 26 and an Al_(0.2)Ga_(0.8)N layer 27. Referring toFIG. 11, in the case where the semiconductor device to be produced is anSBD (Schottky Barrier Diode) which is another example of electronicdevices, the group III nitride layer may be configured to include, forexample, an n⁺-GaN layer 28 (having a carrier concentration for exampleof 2×10¹⁸ cm⁻³) and an n⁻-GaN layer 29 (having a carrier concentrationfor example of 5×10¹⁵ cm⁻³).

Reference Embodiment III-4: Group III Nitride Semiconductor Device

Referring now to FIGS. 9 and 10, a group III nitride semiconductordevice 4 which is a further reference embodiment of Reference InventionIII will be described.

Group III nitride semiconductor device 4 includes group III nitride film13 in the group III nitride composite substrate of Reference EmbodimentIII-1, and at least one group III nitride layer 20 disposed on group IIInitride film 13.

Thus, group III nitride semiconductor device 4 in the present referenceembodiment includes: group III nitride composite substrate 1 whichincludes group III nitride film 13 having a thickness of 10 μm or moreand 250 μm or less; and group III nitride layer 20 with a remarkablyhigh crystal quality grown and thereby disposed on the compositesubstrate, and therefore has excellent semiconductor characteristics.

In group III nitride semiconductor device 4, group III nitride layer 20varies depending on the type of group III nitride semiconductor device4. As shown in FIG. 9, in the case where group III nitride semiconductordevice 4 is a light-emitting device, group III nitride layer 20 may beconfigured to include, for example, an n-GaN layer 21, ann-In_(0.05)Ga_(0.95)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25. As shown in FIG. 10, in the case where group III nitridesemiconductor device 4 is an HEMT which is an example of electronicdevices, group III nitride layer 20 may be configured to include, forexample, a GaN layer 26 and an Al_(0.2)Ga_(0.8)N layer 27, and a sourceelectrode 60, a drain electrode 70, a gate electrode 80, and the likecan be formed on Al_(0.2)Ga_(0.8)N layer 27. As shown in FIG. 11, in thecase where the semiconductor device is an SBD which is another exampleof electronic devices, the group III nitride layer may be configured toinclude, for example, an n⁺-GaN layer 28 (having a carrier concentrationfor example of 2×10¹⁸ cm⁻³) and an n⁻-GaN layer 29 (having a carrierconcentration for example of 5×10¹³ cm⁻³).

As shown in FIGS. 9 and 10, preferably group III nitride semiconductordevice 4 further includes at least one of support substrate 11 and adevice support substrate 40 for supporting group III nitride layer 20.Here, the shape of device support substrate 40 is not limited to theshape of a flat plate, and may be any as long as it supports group IIInitride film 13 and group I nitride layer 20 so that group III nitridesemiconductor device 4 can be formed.

The group III nitride semiconductor device in the reference embodimentas described above can be manufactured by the following manufacturingmethod.

Namely, by the following manufacturing method, group III nitridesemiconductor devices can be manufactured at a low cost and with a highyield, and have remarkably excellent semiconductor characteristics.

Reference Embodiment II-5: Method for Manufacturing Group III NitrideSemiconductor Device

Referring now to FIG. 15, a method for manufacturing a group III nitridesemiconductor device which is a further reference embodiment ofReference Invention III will be described.

The method for manufacturing a group III nitride semiconductor device inthe present reference embodiment includes the steps of: preparing groupIII nitride composite substrate 1; and growing at least one group IIInitride layer 20 on group III-nitride-film 13-side main surface 13 m ofgroup III nitride composite substrate 1.

Here, the step of preparing group III nitride composite substrate 1 issimilar to that of the method for manufacturing a group III nitridecomposite substrate described above in connection with ReferenceEmbodiment III-2. Therefore, the same description will not be repeated.In the following, the steps after the step of preparing group IIInitride composite substrate 1 will be described.

The method for manufacturing a group III nitride semiconductor device inthe present reference embodiment includes, as shown in FIG. 15, the stepof growing at least one group III nitride layer 20 on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1 (FIG. 15 (A)). The method for manufacturing agroup III nitride semiconductor device in the present referenceembodiment can be used to manufacture group III nitride semiconductordevices having excellent characteristics with a high yield, since thegroup III nitride layer is grown on group III-nitride-film 13-side mainsurface 13 m when the group III nitride layer is grown.

The method for manufacturing a group III nitride semiconductor device inthe present reference embodiment can further include the step of furtherbonding device support substrate 40 onto group III nitride layer 20(FIG. 15 (B)), and the step of removing support substrate 11 from groupIII nitride composite substrate 1 (FIG. 15 (C)). These steps can beadded to enable group III nitride semiconductor devices having a highmechanical strength and excellent characteristics and supported bydevice support substrate 40 to be manufactured with a high yield. In thefollowing, each step will be described specifically.

<Step of Growing Group III Nitride Layer>

In the step of growing at least one group III nitride layer 20 on groupIII-nitride-film 13-side main surface 13 m of group III nitridecomposite substrate 1 as shown in FIG. 15 (A), suitable methods forgrowing group III nitride layer 20 are gas phase methods such as MOVPE,MBE, HVPE, and sublimation method, as well as liquid phase methods suchas flux method, in order to epitaxially grow group III nitride layer 20having a high crystal quality, and a particularly suitable method isMOVPE.

The structure of group III nitride layer 20 varies depending on the typeof group III nitride semiconductor device 4. In the case where group IIInitride semiconductor device 4 is a light-emitting device, group IIInitride layer 20 may be configured by successively growing, on group IIInitride film 13, for example, an n-GaN layer 21, ann-In_(0.05)Ga_(0.95)N layer 22, an active layer 23 having a multiplequantum well structure, a p-Al_(0.09)Ga_(0.91)N layer 24, and a p-GaNlayer 25.

In this way, at least one group III nitride layer 20 is grown on groupIII nitride film 13 of group III nitride composite substrate 1, andaccordingly laminated group III nitride composite substrate 2 isobtained.

<Step of Bonding Device Support Substrate>

The step of further bonding device support substrate 40 onto group IIInitride layer 20 as shown in FIG. 15 (B) is performed by forming a firstelectrode 30 and a pad electrode 33 on group III nitride layer 20 oflaminated group III nitride composite substrate 2, forming a padelectrode 43 and a joint metal film 44 on device support substrate 40,and bonding joint metal film 44 to pad electrode 33. Through thesesteps, laminated substrate 3 is obtained. As device support substrate40, Si substrate, CuW substrate, or the like is used.

<Step of Removing Support Substrate>

The step of removing support substrate 11 from group III nitridecomposite substrate 1 as shown in FIG. 15 (C) is performed by removingsupport substrate 11 of group III nitride composite substrate 1 fromlaminated substrate 3. Thus, joint film 12 interposed between supportsubstrate 11 and group III nitride film 13 can simultaneously beremoved.

The method for removing support substrate 11 and joint film 12 is notparticularly limited, and suitable methods to be used are grinding,etching, and the like.

For example, support substrate 11 formed of a material which is low inhardness, strength, and wear resistance, and is likely to be ground off,can be removed by at least one of grinding and polishing, in order toreduce the manufacturing cost. Support substrate 11 formed of a materialwhich can be dissolved in a chemical solution such as acid solution oralkali solution, can be removed through etching with a chemicalsolution, since it requires low manufacturing cost. As support substrate11, a support substrate formed of a polycrystalline material such asceramic material is more preferred as compared with a support substrateformed of a monocrystalline material such as sapphire, SiC, group IIInitride (GaN for example), or the like, since the support substrate 11of the polycrystalline material is easier to remove.

<Step of Forming Electrode>

As shown in FIG. 15 (D), on group III nitride film 13 which has beenexposed after removal of support substrate 11 and joint film 12 fromlaminated substrate 3, a second electrode 50 is formed and, on devicesupport substrate 40, a device support substrate electrode 45 is formed.

In this way, group III nitride semiconductor devices having remarkablyexcellent characteristics can be manufactured with a high yield.

EXAMPLES

Examples 1 to 5 and Comparative Examples 1 to 2 according to the presentinvention will be illustrated below.

Example 1

1. Fabrication of Group III Nitride Composite Substrate

(1) Bonding Group III Nitride Film and Support Substrate

Referring to FIG. 4 (A), group III nitride film 13 and support substrate11 were bonded to each other in the following way.

Referring to FIG. 4 (A1), support substrate 11 was prepared.Specifically, three mullite (Al₂O₃—SiO₂) substrates each having two mainsurfaces both polished into a mirror surface (flat surface having anarithmetic mean roughness Ra defined under JIS B0601 of 1 nm or less,this definition will be applied hereinafter) and having a diameter of10.0 cm and a thickness of 500 μm were prepared. On main surface 11 m ofthis support substrate 11, an SiO₂ film of 1 μm in thickness was formedas joint film 12 a by CVD.

Referring to FIG. 4 (A2), group III nitride film 13 was prepared.Specifically, three different GaN films A to C each having two mainsurfaces both polished into a mirror surface and having a diameter of10.0 cm and a thickness of 400 μm were prepared. GaN film A was notactively doped with a conductivity improver impurity. GaN film B wasdoped with O (oxygen) atoms serving as a conductivity improver impurityat a concentration of 7×10¹⁷ cm⁻³. GaN film C was doped with O (oxygen)atoms serving as a conductivity improver impurity at a concentration of1.6×10¹⁸ cm⁻³.

On main surface 13 n which was a (000-1) plane corresponding to an Natomic plane of group III nitride film 13, an SiO₂ film of 1 μm inthickness was formed as joint film 12 b by CVD.

Referring next to FIG. 4 (A3), each of main surface 12 am of joint film12 a formed on support substrate 11 and main surface 12 bn of joint film12 b formed on group III nitride film 13 was polished into a mirrorsurface, and thereafter main surface 12 am of joint film 12 a and mainsurface 12 bn of joint film 12 b were bonded to each other. Annealingwas performed by raising the temperature to 700° C. in an N₂ gas(nitrogen gas) atmosphere to thereby increase the joint strength.

The three different bonded substrates in which they were thus bonded toeach other were each cut by dicing into eight bonded substrate pieceshaving a main surface of 20 mm×20 mm (square of 20 mm×20 mm, which willalso be applied hereinafter).

(2) Adjustment of Thickness of Group III Nitride Film

Referring to FIG. 4 (B), the thickness of group III nitride film 13 ofthe eight bonded substrate pieces obtained from each of the threedifferent bonded substrates was adjusted by polishing the main surfaceso that respective group III nitride films were 5 μm, 8 μm, 10 μm, 20μm, 50 μm, 100 μm, 200 μm, and 500 μm. Thus, eight group III nitridecomposite substrates were obtained.

2. Measurement of Sheet Resistance of Group III-Nitride-Film-Side MainSurface of Group III Nitride Composite Substrate

For the 24 group III nitride composite substrates 1 obtained from thethree different bonded substrates produced in the above-described manner(eight group III nitride composite substrates per each of the threedifferent bonded substrates), the sheet resistance of groupIII-nitride-film 13-side main surface 13 m was measured by thefour-terminal method. The results are summarized in Table 1.

TABLE 1 group III nitride film sheet resistance type thickness (μm)(Ω/sq) GaN film 5 403.6 A 8 250.8 10 196.1 20 101.6 50 41.5 100 19.8 2009.7 500 3.8 GaN film 5 157.7 B 8 101.3 10 78.9 20 39.2 50 18.1 100 7.4200 3.7 500 1.3 GaN film 5 40.7 C 8 24.5 10 19.4 20 9.7 50 3.6 100 1.7200 0.9 500 0.3

3. Fabrication of Group III Nitride Semiconductor Device

Referring to FIG. 5, on main surface 13 m which was a {0001} planecorresponding to a group III atomic plane of group III nitride film 13in group III nitride composite substrate 1, group III nitride layer 20was grown by MOCVD. Specifically, an n-type GaN layer having a thicknessof 5 μm which was first-conductivity-type GaN layer 201, an n-typeAl_(0.05)Ga_(0.95)N layer having a thickness of 0.5 μm which wasfirst-conductivity-type Al_(s)Ga_(1-s)N layer 202 (here, s meets 0<s<1),an MQW (Multiple Quantum Well) structure layer having a thickness of 100nm and made up of six cycles of In_(0.15)Ga_(0.85)N layer andIn_(0.01)Ga_(0.99)N layer which was light-emitting layer 203, a p-typeAl_(0.20)Ga_(0.80)N layer having a thickness of 20 nm which wassecond-conductivity-type Al_(t)Ga_(1-t)N layer 204 (here, t meets0<t<1), and a p-type GaN layer having a thickness of 0.15 μm which wassecond-conductivity-type GaN layer 205 were grown in this order.

Next, second-conductivity-type GaN layer 205, second-conductivity-typeAl_(t)Ga_(1-t)N layer 204, light-emitting layer 203, andfirst-conductivity-type Al_(s)Ga_(1-s)N layer 202 were each partiallyremoved by mesa etching to expose a part of first-conductivity-type GaNlayer 201.

Next, on the exposed main surface of first-conductivity-type GaN layer201, an n-side electrode was formed as first electrode 31 by electronbeam vapor deposition. In addition, on the exposed main surface ofsecond-conductivity-type GaN layer 205, a p-side electrode was formed assecond electrode 32 by electron beam vapor deposition. In this way, 24group III nitride semiconductor devices were fabricated.

A reference group III nitride semiconductor device was prepared in thefollowing way. On main surface 13 m of the GaN substrate that was a(0001) plane corresponding to a group III atomic plane, namely Ga atomicplane, group III nitride layer 20 of the same structure as theabove-described one was formed in a similar manner to theabove-described one. The GaN substrate had the two main surfaces bothpolished into a mirror surface and having a size of 20 mm×20 mm and athickness of 500 μm, and was doped with a conductivity improverimpurity, namely O (oxygen) atoms, at a concentration of 1.6×10¹⁸ cm⁻³.

4. Measurement of Emission Intensity of Group III Nitride SemiconductorDevice

The emission intensity of the 24 group III nitride semiconductor devicesand the reference group III nitride semiconductor device obtained asdescribed above was measured in the following way. Current of 80 mA wasapplied to the group III nitride semiconductor device. The emissionintensity at a peak wavelength of 450 nm of the emission spectrum atthis time was measured by the EL (Electro Luminescence) method. Theratio of the emission intensity of each of the 24 group III nitridesemiconductor devices, relative to the emission intensity of thereference group III nitride semiconductor device, was calculated for useas a relative emission intensity. A light-emitting device having a groupIII nitride composite substrate including GaN film A of 5 μm inthickness and having a sheet resistance of 403.6 Ω/sq, and alight-emitting device having a group III nitride composite substrateincluding GaN film A of 8 μm in thickness and having a sheet resistanceof 250.8 Ω/sq each had a low relative emission intensity of 0.01 orless. The relative emission intensity of light-emitting devices havingthe other group III nitride composite substrates had a higher relativeemission intensity of 0.1 or more.

Example 2

As group III nitride films 13, 20 GaN films having a diameter of twoinches (5.08 cm) and a thickness of 500 μm, doped with O (oxygen) atomswhich were a conductivity improver impurity at a concentration of 7×10¹⁷cm⁻³, and having two main surfaces both polished into a mirror surfacewere prepared.

Of these GaN films, ten GaN films were polished so that the thicknesswas reduced to 200 μm. As a result, breakage and/or cracks occurred toseven GaN films out of the ten GaN films.

To the remaining ten GaN films each, a mullite substrate having adiameter of two inches (5.08 cm) and a thickness of 400 μm was bondedthrough a similar procedure to Example 1, and thereafter the bonded GaNfilm was polished so that the thickness was reduced to 200 μm. Then, nobreakage and/or cracks occurred to all of the ten GaN films.

Example 3

Six group III nitride composite substrates 1 were fabricated through asimilar procedure to Example 1, except that a GaN film having a diameterof two inches (5.08 cm) and a thickness of 500 μm, doped with noconductivity improver impurity, and having two main surfaces bothpolished into a mirror surface was used as group III nitride film 13,and that a watermark was formed on the surface to be bonded to therebychange the area of the bonded region. Here, the watermark is a dry stainformed as follows. As moisture in a portion of a surface where animpurity is present due to cleaning contamination or the like is dried,the portion is dried with the impurity aggregated, and accordingly thedry stain is formed. The watermark can be formed by wetting a surfacewhere the watermark is to be formed, with low-purity water rather thanultrapure water and drying it to thereby form the watermark in theportion wet with the low-impurity water.

For the obtained six group III nitride composite substrates 1, the areaof joined region 100 b was measured with a surface defect inspectionapparatus using a laser. As a result, respective areas of the joinedregions of the substrates relative to the area of main surface 11 m were34%, 57%, 64%, 70%, 87%, and 95%. Of these group IlI nitride compositesubstrates, group III nitride composite substrates 1 in which the areaof joined region 100 b was 70% or more relative to the area of mainsurface 1 m had non-joined partial regions constituting non-joinedregion 100 n that all had a maximum size in radial direction of lessthan 20 mm. As for group III nitride composite substrates 1 in which thearea of joined region 100 b was 64% or less relative to the area of mainsurface 1 m, some non-joined partial regions constituting non-joinedregion 100 n had a maximum size in radial direction of 20 mm or more.Moreover, group III nitride composite substrates 1 in which the area ofjoined region 100 b was 64% or less relative to the area of main surface11 m were heated to 700° C. As a result, group III nitride film 13 waspeeled off during the heating, from support substrate 11 from thenon-joined partial region with a maximum size in radial direction of 20mm or more.

Example 4

Three group III nitride composite substrates 1 were fabricated through asimilar procedure to Example 1, except that: a GaN film having adiameter of two inches (5.08 cm) and a thickness of 500 μm, doped withno conductivity improver impurity, and having two main surfaces bothpolished into a mirror surface was used as group III nitride film 13; apolycrystalline Mo substrate having a diameter of two inches (5.08 cm)and a thickness of 500 μm and having two main surfaces both polishedinto a mirror surface was used as support substrate 11; the surfaces tobe bonded to each other were cleaned in the process of bonding tothereby reduce generation of non-joined regions; and a water mark havinga diameter on the order of 15 mm was formed on the surface to be bondedto thereby form a non-joined region at a predetermined position of thegroup III nitride composite substrate.

For the obtained three group III nitride composite substrates 1, theposition of the non-joined region was measured with an ultrasonicmicroscope. As a result, in one group III nitride composite substrate,there was formed the non-joined region including a non-joined partialregion having a maximum size in radial direction on the order of 10 mmand abutting on the perimeter of the main surface. In another group IIInitride composite substrate, there was formed the non-joined regionincluding only a non-joined partial region having a maximum size inradial direction on the order of 10 mm and failing to abut on theperimeter of the main surface. In the remaining one group III nitridecomposite substrate, the area of the non-joined region was 83% relativeto the area of the main surface.

The obtained three group III nitride composite substrates were heated to700° C. As a result, the group III nitride composite substrate in whichthere was formed the non-joined region including the non-joined partialregion which had a maximum size in radial direction on the order of 10mm and abutted on the perimeter of the main surface was broken duringthe heating. The remaining group III nitride composite substrates werenot broken.

Example 5

As group III nitride films, eight GaN films whose two main surfaces wereboth polished into a mirror surface, main surface was 20 mm×20 mm, andthickness was 300 μm were prepared. The eight GaN films were dividedinto four sets each including two GaN films, and main-surface throughholes having a diameter on the order of 50 μm were formed so that thearea of the main-surface through holes relative to the area of the mainsurface were 5%, 10%, 20%, and 30%, respectively in these four sets.

Moreover, eight substrates were prepared in which the support substratewas a sapphire substrate whose main surface was 20 mm×20 mm andthickness was 400 μm and an SiO₂ film was formed as the joint film onthe main surface of the sapphire substrate. The main surface of thejoint film was polished into a mirror surface. After this, four of theeight substrates were cleaned multiple times in an aqueous solutioncontaining 37% by mass of HCl (hydrochloric acid) and 30% by mass ofH₂O₂ (hydrogen peroxide), to thereby remove an impurity containing metalon the surface to be bonded. The remaining four substrates were cleanedwith IPA (isopropyl alcohol).

The four group III nitride films in which respective areas of themain-surface through holes were 5%, 10%, 20%, and 30% relative to thearea of the main surface, and the four support substrates in which thesurface of the joint film to be bonded was cleaned with HCl and H₂O₂were bonded to each other under similar conditions to Example 1.Further, the four group III nitride films in which respective areas ofthe main-surface through holes were 5%, 10%, 20%, and 30% relative tothe area of the main surface and the four support substrates in whichthe surface of the joint film to be bonded was cleaned with IPA werebonded to each other under similar conditions to Example 1.

After they were thus bonded to each other, a tensile test was performedin the following way. The tensile test was performed by attaching ajigof 10 mm×10 mm with an epoxy adhesive to the front and rear sides of thegroup III nitride composite substrate and pulling both ends of the jig.The results are summarized in Table 2. In Table 2, the compositesubstrates on which fracture occurred between the epoxy adhesive and thejig exhibit a higher joint strength than those in which the group IIInitride film and the joint film are separated from each other alongtheir interface.

TABLE 2 area (%) of main- method for surface through hole cleaning ingroup III nitride surface to film relative to area region of fractureand form of be bonded of main surface fracture under tensile testcleaning with 5 fracture along interface between HCl and H₂O₂ epoxyadhesive and jig 10 separation along interface between group III nitridefilm and joint film 20 separation along interface between group IIInitride film and joint film 30 separation along interface between groupIII nitride film and joint film cleaning with 5 fracture along interfacebetween IPA epoxy adhesive and jig 10 fracture along interface betweenepoxy adhesive and jig 20 separation along interface between group IIInitride film and joint film 30 separation along interface between groupIII nitride film and joint film

Referring to Table 2, the composite substrates which were cleaned withIPA had a higher joint strength where the area of the main-surfacethrough hole of the group III nitride film was 10% or less relative tothe area of the main surface. In addition, the concentration of animpurity containing metal on the surface to be bonded of the joint filmcleaned with HCl and H₂O₂ and that of the joint film cleaned with IPAwere measured by TXRF (total reflection x-ray fluorescence) analysis.Those cleaned with HCl and H₂O₂ had an Fe concentration and an Niconcentration which were both 1×10¹⁰ cm⁻² or less. Those cleaned withIPA had an Fe concentration of 5.7×10¹¹ cm⁻² and an Ni concentration of1.5×10¹¹ cm².

Comparative Example 1

As the group III nitride film, one GaN film having a diameter of twoinches (5.08 cm) and a thickness of 400 μm, doped with O (oxygen) atomsat a concentration of 6×10⁷ cm⁻³ which was a conductivity improverimpurity, and having two main surfaces both polished into a mirrorsurface was prepared. In a main surface, which was an N atomic surfaceof the group III nitride film, H (hydrogen) ions were implanted to aposition at a depth of about 0.7 μm from the main surface. The H ionswere implanted with an acceleration voltage of 100 keV and a dose of5×10¹⁷ cm⁻². The main surface, where the H ions were implanted, of thegroup III nitride film implanted with the H ions, and a mullitesubstrate having a diameter of two inches (5.08 cm) and a thickness of500 μm, were bonded to each other and annealed through a similarprocedure to Example 1. Accordingly, the group III nitride film wasseparated along the region where the H ions were implanted, so that agroup III nitride composite substrate in which a group III nitride filmof 0.3 μm in thickness is formed on the mullite substrate was obtained.The sheet resistance of the obtained group III nitride compositesubstrate was 1×10¹⁰ f/sq or more.

Comparative Example 2

A group III nitride composite substrate having a group III nitride filmof 0.3 μm in thickness on an Mo substrate was produced through a similarprocedure to Comparative Example 1, except that the Mo substrate havinga diameter of two inches (5.08 cm) and a thickness of 300 μm and havingtwo main surfaces both polished into a mirror surface was used as thesupport substrate, and that the dose of H ions was 3.5×10¹⁷ cm⁻². Theobtained group III nitride composite substrate was further heat-treatedat 800° C. for three hours in an N₂ gas (nitrogen gas) atmosphere.Accordingly, the sheet resistance was reduced to 6700 Ω/sq in most ofthe region, while the sheet resistance was still a high resistance of35000 Ω/sq in a part of the region.

Reference Example I-A to Reference Example I-I according to ReferenceInvention I will be illustrated below.

Reference Example I-A

1. Fabrication of Group III Nitride Composite Substrate

Referring to FIG. 12 (A), a mullite substrate having a diameter of 75 mmwas prepared for use as support substrate 11. Both surfaces of supportsubstrate 11 were subjected to rough polishing by means of diamondabrasive grains and a copper-based surface plate, intermediate polishingby means of diamond abrasive grains and a tin surface plate, and finishpolishing by means of a nonwoven polishing pad so that the surfaces weremirror-finished to have an RMS (root mean square roughness) of 5 nm orless. After this, an SiO₂ film was grown on this to a thickness of 800nm by the LP-CVD (Low Pressure-Chemical Vapor Deposition) method. Then,CMP was performed using a slurry having a pH of 10 and containingcolloidal silica abrasive grains having an average grain size of 40 nm,to thereby form joint film 12 a having a thickness of 400 nm andplanarized so that the RMS of the main surface was 0.3 nm or less. Inorder to remove abrasive grains used for CMP, non-abrasive polishingcleaning with a KOH aqueous solution and cleaning with pure water wereperformed.

Referring to FIG. 12 (B), a GaN crystal body having a diameter of 75 mmand a thickness of 8 mm was prepared for use as group III nitride filmdonor substrate 13D.

A surface-to-be-bonded of group III nitride film donor substrate 13D wassubjected to mechanical polishing and CMP so that it was mirror-finishedto have an RMS of 2 nm or less. After this, an SiO₂ film having athickness of 800 nm was grown by the LP-CVD (Low Pressure-Chemical VaporDeposition) method on the surface-to-be-bonded, and CMP was performedusing a slurry having a pH of 10 and containing colloidal silicaabrasive grains having an average grain size of 40 nm, to thereby formjoint film 12 b having a thickness of 500 nm and having a main surfaceplanarized to have an RMS of 0.3 nm or less. In order to remove abrasivegrains used for CMP, non-abrasive polishing cleaning with a KOH aqueoussolution and cleaning with pure water were performed. Here, group IIInitride film donor substrate 13D was produced through growth by the HVPEmethod using a GaAs substrate as a base substrate.

Referring to FIG. 12 (C), joint film 12 a and joint film 12 b werebonded together to thereby produce joined substrate 1L in which supportsubstrate 11 and group III nitride film 13 were bonded to each otherwith joint film 12 interposed therebetween. After they were bondedtogether, joined substrate 1L was annealed by being heated in a nitrogengas atmosphere to 800° C., to thereby increase the joint strength.

Referring to FIG. 12 (D), group III nitride film donor substrate 13D injoined substrate L was cut, with a wire saw, along a plane locatedinwardly at a depth of 180 μm from the bonded surface, namely thesurface of group III nitride film donor substrate 13D bonded to jointfilm 12, to thereby produce group III nitride composite substrate 1 inwhich support substrate 11 and the GaN film which was group III nitridefilm 13 were bonded together with joint film 12 interposed therebetween.As the wire, a fixed-abrasive wire on which diamond abrasive grains wereelectrodeposited was used. As for the cutting method, in order to reducethe cut resistance and enhance the thickness precision and the flatness,the method was used that caused the wire to swing and caused group IIInitride film donor substrate 13D to vibrate in synchronizationtherewith. The resistance coefficient for cutting with the wire saw wasset to 4200 N.

After cutting, group III nitride film 13 of group III nitride compositesubstrate 1 was subjected to mechanical polishing and CMP. In order tohave a uniform thickness and a uniform off angle of group III nitridefilm 13, the composite substrate was mounted on a CMP apparatus in thefollowing way. The shape of the substrate was corrected in advance byvacuum chuck suction, and thereafter the composite substrate wassuction-fixed onto the apparatus.

Regarding group III nitride composite substrate 1 thus produced, theratio s_(t)/m_(t) of standard deviation s, of the thickness of group IIInitride film 13, to mean value m_(t) of the thickness thereof, and theratio s_(o)/m_(o) of standard deviation s_(o) of an absolute value of anoff angle between the main surface of the group III nitride film and the(0001) plane, to mean value m_(o) of the absolute value of the off anglethereof are shown in Table 3. Here, mean value m_(t) of the thickness,standard deviation s_(t) of the thickness, mean value m_(o) of theabsolute value of the off angle, and standard deviation s_(o) of theabsolute value of the off angle, were calculated from the thickness andthe absolute value of the off angle of group III nitride film 13 at the13 measurement points P on main surface 13 m of group III nitride film13 as shown in FIG. 7, constituted of: one central point P_(C); fourouter points P_(O) located respectively in the four directions withrespect to central point P_(C) that are orthogonal to each other, theouter points each being located at 5 mm inward from the outer edge ofthe substrate; and eight middle points P_(M) including four middlepoints each between the one central point P_(C) and one of the fourouter points P_(O) and four middle points each between two of the fourouter points P_(O).

2. Fabrication of Group III Nitride Semiconductor Device

Referring to FIG. 15 (A), on group III nitride film 13 of group ITnitride composite substrate 1, group III nitride layer 20 was formed bythe MOVPE method. Specifically, on group III nitride film 13, n-GaNlayer 21 having a thickness of 5 μm, n-In_(0.05)Ga_(0.95)N layer 22having a thickness of 50 nm, active layer 23 having a multiple quantumwell structure of three cycles constituted of an In_(0.14)Ga_(0.86)Nwell layer having a thickness of 3 nm and a GaN barrier layer having athickness of 15 nm, p-Al_(0.09)Ga_(0.91)N layer 24 having a thickness of20 nm, and p-GaN layer 25 having a thickness of 150 nm were successivelyepitaxially grown to thereby produce laminated group III nitridecomposite substrate 2. After this, it was annealed by an RTA (RapidThermal Annealing) apparatus and accordingly activated.

Referring to FIG. 15 (B), on p-GaN layer 25, which was the topmost layerin group III nitride layer 20 of laminated group III nitride compositesubstrate 2, an Ni layer having a thickness of 4 nm and an Au layerhaving a thickness of 200 nm were successively formed by the EB(Electron Beam) deposition method, and annealed into an alloy, tothereby form first electrode 30. On first electrode 30, a Ti layerhaving a thickness of 200 nm, a Pt layer having a thickness of 100 nm,and an Au layer having a thickness of 1000 nm were successively formedby the EB deposition method to thereby form pad electrode 33.

A CuW substrate was prepared for use as device support substrate 40. Ondevice support substrate 40, a Ti layer having a thickness of 200 nm, aPt layer having a thickness of 100 nm, and an Au layer having athickness of 1000 nm were successively formed by the EB depositionmethod to thereby form pad electrode 43. On pad electrode 43, an AuSnsolder film was formed as joint metal film 44.

Subsequently, joint metal film 44 was bonded to pad electrode 33 tothereby produce laminated substrate 3.

Referring to FIG. 15 (C), from laminated substrate 3, support substrate11 and joint film 12 in group III nitride composite substrate 1 wereetched away by means of hydrofluoric acid.

Referring to FIG. 15 (D), on group III nitride film 13 having beenexposed by removal of support substrate 11 and joint film 12 fromlaminated substrate 3, a Ti layer having a thickness of 20 nm, an Allayer having a thickness of 200 nm, and an Au layer having a thicknessof 300 nm were successively formed by the EB deposition method, and thenannealed to form second electrode 50. On device support substrate 40, aTi layer having a thickness of 20 nm and an Au layer having a thicknessof 300 nm were successively formed by the EB deposition method, andannealed to thereby form device support substrate electrode 45. In thisway, group III nitride semiconductor device 4 was obtained.

For group III nitride semiconductor device 4 thus obtained, its opticaloutput was measured by means of an integrating sphere under thecondition that injected current was 4 A. The optical output of thelight-emitting device was measured in the following way. Specifically,into the light-emitting device mounted in the integrating sphere,predetermined current was injected, and the optical output was measuredby a detector receiving the light collected from the light-emittingdevice. The resultant group III nitride semiconductor devices wereclassified into non-defective devices meeting a standard that theoptical output was 2 W or more and defective devices failing to meetthis standard, and the percentage of the ratio determined by dividingthe non-defective devices by the sum of the non-defective devices andthe defective devices was defined as a yield. The yield of the group IIInitride semiconductor devices is summarized in Table 3.

TABLE 3 Reference Example I-A I-A1 I-A2 I-A3 I-A4 I-A5 I-A6 I-A7 I-A8I-A9 ratio s_(t)/m_(t) 0.001 0.002 0.01 0.05 0.1 0.15 0.2 0.25 0.2 ratios_(o)/m_(o) 0.005 0.008 0.05 0.2 0.4 0.5 0.6 0.6 0.7 device yield (%) 7877 75 72 68 62 58 35 32

Referring to Table 3, group III nitride semiconductor devicesmanufactured with a high yield were those using a group III nitridecomposite substrate having a diameter of 75 mm and including a group IIInitride film having a thickness of 150 μm, where the ratio si/me ofstandard deviation s, of the thickness of the group III nitride film, tomean value m_(t) of the thickness thereof was 0.001 or more and 0.2 orless, and the ratio s_(o)/m_(o) of standard deviation s_(o) of anabsolute value of an off angle between the main surface of the group IIInitride film and the {0001} plane, to mean value m_(o) of the absolutevalue of the off angle thereof was 0.005 or more and 0.6 or less. Itshould be noted that, as to Reference Example I-A1, sophisticatedcontrol was necessary for cutting and polishing in order to reduce thethickness variation and the off-angle variation, and thus the cuttingand polishing took a long time.

Reference Example I-B

Referring to FIGS. 12 and 15, group III nitride composite substrate 1and group III nitride semiconductor device 4 were produced in a similarmanner to Reference Example I-A, except that a mullite-YSZ substrate(with respect to the whole substrate, mullite was 70% by mass and YSZwas 30% by mass and, with respect to YSZ, ZrO₂ was 90% by mole and Y₂O₃was 10% by mole) was used as support substrate 11, and group III nitridecomposite substrates 1 having a diameter of 75 mm, a diameter of 100 mm,a diameter of 125 mm, and a diameter of 150 m, respectively, wereproduced.

In a similar manner to Reference Example I-A, the ratio s_(t)/m_(t) ofstandard deviation s_(t) of the thickness of group III nitride film 13,to mean value m, of the thickness thereof, and the ratio s_(o)/m_(o) ofstandard deviation so of an absolute value of an off angle between themain surface of the group III nitride film and the (0001) plane, to meanvalue m_(o) of the absolute value of the off angle thereof werecalculated for group III nitride composite substrate 1. The results aresummarized in Table 4. The yield of group III nitride semiconductordevices 4 was also calculated in a similar manner to Reference ExampleI-A. The results are summarized in Table 4.

TABLE 4 Reference Example I-B I-B1 I-B2 I-B3 I-B4 I-B5 I-B6 I-B7 I-B8I-B9 I-B10 I-B11 I-B12 I-B13 I-B14 diameter (mm) 75 75 75 75 75 100 100100 100 125 150 150 75 100 thickness of group 10 30 100 170 200 10 110170 200 110 120 160 250 250 III nitride film (μm) ratio s_(t)/m_(t) 0.120.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.12 ratios_(o)/m_(o) 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3 0.3device yield (%) 60 63 70 72 61 59 71 73 60 72 71 72 59 58

Referring to Table 4, group III nitride semiconductor devicesmanufactured with a high yield were those using a group III nitridecomposite substrate having a diameter of 75 mm to 150 mm and including agroup III nitride film having a thickness of 10 μm to 250 μm, where theratio s_(t)/m_(t) of standard deviation s_(t) of the thickness of thegroup III nitride film, to mean value m_(t) of the thickness thereof was0.12 falling within a range of 0.001 or more and 0.2 or less, and theratio s_(o)/m_(o) of standard deviation s_(o) of an absolute value of anoff angle between the main surface of the group III nitride film and the(0001) plane, to mean value m_(o) of the absolute value of the off anglethereof was 0.3 falling within a range of 0.005 or more and 0.6 or less.

Reference Example I-C

As substrates for semiconductor devices, a group III nitridefree-standing substrate (hereinafter also referred to as FS substrate),a group III nitride composite substrate (hereinafter also referred to asBP substrate) produced by the ion implantation method, and a group IIInitride composite substrate (hereinafter also referred to as BSsubstrate) produced in accordance with Reference Embodiment I-4 of thepresent invention were prepared.

The FS substrate was prepared to have the diameter and the thicknessshown in Table 5, by cutting a GaN crystal body having a predetermineddiameter with a wire saw and polishing it.

The BP substrate was prepared to have the diameter and the thickness ofits group III nitride film shown in Table 5. Specifically, as shown inFIG. 17 (B), hydrogen ions were implanted from the main surface of theGaN crystal body having a predetermined diameter, namely group IIInitride film donor substrate 13D, to the position of a predetermineddepth located inwardly from the main surface, to thereby form an ionimplantation region 13 i. After this, as shown in FIG. 17 (C), supportsubstrate 11 and the ion implantation region 13 i side of group IIInitride film donor substrate 13D were bonded together with joint film 12interposed therebetween. After this, as shown in FIG. 17 (D), annealingwas done at 850° C. to separate group III nitride film donor substrate13D along its ion implantation region 13 i. Here, a mullite substratewas used as support substrate 11.

The BS substrate was prepared to have the diameter and the thickness ofits group III nitride film as shown in Table 5, in a similar manner toReference Example 1-B except that a mullite substrate was used as thesupport substrate.

Group III nitride composite substrates 1 and group III nitridesemiconductor devices 4 were fabricated in a similar manner to ReferenceExample I-B, except that the above-described FS substrate, BP substrate,and BS substrate were used.

In a similar manner to Reference Example I-A, the ratio s_(t)/m_(t) ofstandard deviation s_(t) of the thickness of group III nitride film 13,to mean value m_(t) of the thickness thereof, and the ratio s_(o)/m_(o)of standard deviation s_(o) of an absolute value of an off angle betweenthe main surface of the group III nitride film and the (0001) plane, tomean value m_(o) of the absolute value of the off angle thereof werecalculated for group III nitride composite substrate 1. The results aresummarized in Table 5. The yield of group III nitride semiconductordevices 4 was also calculated in a similar manner to Reference ExampleI-A. The results are summarized in Table 5.

TABLE 5 FS substrate BP substrate BS substrate Reference Example I-CI-C1 I-C2 I-C3 I-C4 I-C5 I-C6 I-C7 I-C8 I-C9 I-C10 I-C11 I-C12 I-C13I-C14 diameter (mm) 50 75 75 100 100 100 125 75 100 125 75 100 125 150thickness of 250 200 400 250 450 450 500 0.5 0.5 0.5 150 150 150 150group III nitride film (μm) warp (μm) 50 140 40 180 30 30 35 25 30 35 3030 30 30 ratio s_(t)/m_(t) 0.16 0.16 0.16 0.16 0.16 0.25 0.16 0.16 0.160.16 0.16 0.16 0.16 0.16 ratio s_(o)/m_(o) 0.42 0.42 0.42 0.42 0.42 0.650.42 0.42 0.42 0.42 0.42 0.42 0.42 0.42 device yield (%) 54 crack 55crack 58 19 57 0 0 0 66 65 67 68

Referring to Table 5, as to the group III nitride semiconductor devicesfabricated using FS substrates having a diameter of 50 mm to 125 mm anda thickness of 200 μm to 500 μm, those having a relatively largediameter and a relatively small thickness had a large warp and werelikely to crack, and the yields of them were all less than 60%.

As to the group III nitride semiconductor devices fabricated using BPsubstrates having a diameter of 75 mm to 125 mm and a thickness of thegroup III nitride film of 0.5 μm, they did not exhibit excellent devicecharacteristics due to the relatively small thickness of the group IIInitride film, resulting in lower yields.

In contrast, the yields of the group III nitride semiconductor devicesproduced using BS substrates were higher, namely 65% or more.

Reference Example 1-D

A plurality of group III nitride composite substrates were fabricated ina similar manner to Reference Example 1-B, except that an Al₂O₃—SiO₂composite oxide substrate (with respect to the whole substrate, Al₂O₃was 85% by mass and SiO₂ was 15% by mass) was used as the supportsubstrate. The warp and the TTV of the substrates were measured. Theresults are shown in Table 6. Here, the warp and the TTV of the groupIII nitride composite substrates were measured by means of an opticalinterferometric flatness tester.

These group III nitride composite substrates were used to fabricategroup III nitride semiconductor devices in a similar manner to ReferenceExample I-B.

For the group III nitride composite substrates, the ratio s_(t)/m_(t) ofstandard deviation s_(t) of the thickness of the group III nitride film,to mean value m_(t) of the thickness thereof, and the ratio s_(o)/m_(o)of standard deviation s_(o) of an absolute value of an off angle betweenthe main surface of the group III nitride film and the (0001) plane, tomean value m_(o) of the absolute value of the off angle thereof, as wellas the warp and the TTV of the group III nitride composite substratesare shown in Table 6. The yield of group III nitride semiconductordevices 4 was also calculated in a similar manner to Reference ExampleI-A. The results are summarized in Table 6.

TABLE 6 Reference Example I-D I-D1 I-D2 I-D3 I-D4 I-D5 I-D6 I-D7 I-D8I-D9 I-D10 I-D11 I-D12 I-D13 diameter (mm) 100 100 100 100 100 100 100100 75 150 150 150 150 thickness of 110 110 110 110 110 110 110 110 130120 120 120 120 group III nitride film (μm) warp (μm) 15 30 50 70 30 3050 50 50 50 50 50 90 TTV (μm) 15 15 20 40 10 20 30 50 30 30 60 30 30ratio s_(t)/m_(t) 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.08 0.080.08 0.08 ratio s_(o)/m_(o) 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.36 0.360.36 0.36 0.36 0.36 device yield (%) 75 68 63 55 74 67 60 57 61 61 58 6057

Referring to Table 6, the group III nitride semiconductor devicesmanufactured with a high yield were those using group III nitridecomposite substrates having a warp of the main surface on the group IIInitride film side of 50 μm or less and having a TTV of 30 μm or less.

Reference Example I-E

Group III nitride semiconductor devices were fabricated in a similarmanner to Reference Example I-B, except that different types of groupIII nitride composite substrates were used that were different in ratioα_(III-N)/α_(S) of thermal expansion coefficient α_(III-N) of the groupIII nitride film to thermal expansion coefficient α_(S) of the supportsubstrate, and ratio t_(III-N)/t_(S) of thickness t_(III-N) of the groupIII nitride film to thickness t_(S) of the support substrate. Here, inorder to vary these ratios, mullite substrate, mullite-YSZ substrate,and Al₂O₃—SiO₂ composite oxide substrate having different thicknesseswere used as base substrates.

The ratio α_(III-N)/α_(S) of thermal expansion coefficient α_(III-N) ofthe group III nitride film to thermal expansion coefficient α_(S) of thesupport substrate, the ratio t_(III-N)/t_(S) of thickness t_(III-N) ofthe group III nitride film to thickness t_(S) of the support substrate,and the yield of the group III nitride semiconductor devices aresummarized in Table 7.

TABLE 7 Reference Example I-E I-E1 I-E2 I-E3 I-E4 I-E5 I-E6 I-E7 I-E8I-E9 I-E10 I-E11 diameter (mm) 75 75 75 75 75 100 100 100 100 150 150type of support mullite mullite mullite mullite mullite mullite mullite-Al₂O₃—SiO₂ Al₂O₃—SiO₂ mullite mullite- substrate YSZ YSZ ratioα_(III-N)/α_(S) 1.2 1.2 1.2 1.2 1.2 1.2 1 1.25 0.75 1.2 1 ratiot_(III-N)/t_(S) 0.02 0.1 0.25 0.6 1 0.3 0.3 0.3 0.3 0.2 0.2 device yield(%) 62 65 70 66 64 71 75 65 62 68 72

Referring to Table 7, the group III nitride semiconductor devicesmanufactured with a high yield were those using group III nitridecomposite substrates having a ratio α_(III-N)/α_(S) of thermal expansioncoefficient α_(III-N) of the group III nitride film to thermal expansioncoefficient α_(s) of the support substrate of 0.75 or more and 1.25 orless, and a ratio t_(III-N)/t_(S) of thickness t_(III-N) of the groupIII nitride film to thickness t_(S) of the support substrate of 0.02 ormore and 1 or less.

Reference Example I-F

Group III nitride composite substrates and group III nitridesemiconductor devices were produced in a similar manner to ReferenceExample I-B, except that the cleaning conditions were tailored to adjustthe amount of impurity metal atoms of the surface of the group IIInitride film. The diameter of the group III nitride composite substrate,the amount of impurity metal atoms of the surface of the group IIInitride film, and the yield of the group III nitride semiconductordevices are summarized in Table 8. Here, the amount of impurity metalatoms of the surface of the group III nitride film was measured by theTXRF (Total Reflection X-ray Fluorescence) method. Here, themeasurements by the TXRF method were done by means of a tungsten (W)x-ray source under a condition that the angle of incidence was 0.05°.

TABLE 8 Reference Example I-F I-F1 I-F2 I-F3 I-F4 I-F5 I-F6 I-F7diameter 75 75 75 75 100 100 150 impurity metal atoms 6 40 300 500 8 8010 (×10¹⁰ atoms/cm²) device yield (%) 70 68 63 58 69 65 69

Referring to Table 8, group III nitride semiconductor devicesmanufactured with a high yield were those using group III nitridecomposite substrates having impurity metal atoms of the main surface ofthe group III nitride film of 3×10¹² atoms/cm² or less.

Reference Example I-G

Group III nitride composite substrates and group III nitridesemiconductor devices were fabricated in a similar manner to ReferenceExample I-B, except that a mullite substrate was used as the supportsubstrate and, as the group III nitride film donor substrate, a uniformGaN crystal body without dislocation concentrated region that had O andSi added as dopants and had a dislocation density of 4×10⁶ cm⁻² and acarrier concentration of 2×0¹⁸ cm⁻³ was used. Here, the polishingconditions were tailored to adjust the maximum RMS of respective mainsurfaces of the group III nitride film and the support substrate.

As to the group III nitride composite substrates, the maximum RMS of themain surface of the group III nitride film and the maximum RMS of themain surface of the support substrate, as well as the yield of the groupIII nitride semiconductor devices are summarized in Table 9.

TABLE 9 Reference Example I-G I-G1 I-G2 I-G3 I-G4 I-G5 I-G6 I-G7 I-G8I-G9 maximum RMS of main 0.5 1 3 5 1 1 1 1 3 surface of group IIInitride film (nm) maximum RMS of main 1 1 1 1 2 6 12 18 12 surface ofsupport substrate (nm) device yield (%) 68 64 58 52 65 63 57 53 55

Referring to Table 9, group III nitride semiconductor devicesmanufactured with a high yield were those using group III nitridecomposite substrates having an RMS of the main surface of the group IIInitride film of 3 nm or less and an RMS of the main surface of thesupport substrate of 12 nm or less.

Reference Example I-H

Group III nitride composite substrates were fabricated in a similarmanner to Reference Example I-B, except that an Al₂O₃—SiO₂ compositeoxide substrate (with respect to the whole substrate, Al₂O₃ was 82% bymass and SiO₂ was 18% by mass) was used as the support substrate and, asthe group III nitride film donor substrate, a semi-insulating GaNcrystal body having Fe added as a dopant and a specific resistance of1×10⁷ Ωcm was used. Here, the polishing conditions were tailored toadjust mean value m_(III-N) and standard deviation s_(III-N) of the RMSof the main surface of the group III nitride film in the group IIInitride composite substrate as well as mean value ms and standarddeviation s_(S) of the RMS of the main surface of the support substrate.

Further, referring to FIG. 10, an HEMT was fabricated as group IIInitride semiconductor device 4 in the following manner.

On group III nitride film 13 of group III nitride composite substrate 1,GaN layer 26 having a thickness of 1.5 μm and Al_(0.2)Ga_(0.8)N layer 27having a thickness of 30 nm were successively epitaxially grown by theMOVPE method to form group III nitride layer 20, to thereby producelaminated group III nitride composite substrate 2.

Next, on Al_(0.2)Ga_(0.8)N layer 27, a source electrode 60 and a drainelectrode 70 were fabricated by photolithography, EB deposition, andlift-off. In these electrodes, a Ti layer having a thickness of 20 nm,an Al layer having a thickness of 100 nm, a Ti layer having a thicknessof 20 nm, and an Au layer having a thickness of 300 nm were successivelyformed, lifted off, and annealed for 1 minute at 600° C. into an alloy.

Next, in a similar step to the step of fabricating source electrode 60and drain electrode 70, a gate electrode 80 was fabricated. Forfabrication of gate electrode 80, an Ni layer having a thickness of 50nm and an Au layer having a thickness of 500 nm were successivelyformed. The gate length was set to 2 μm.

In the HEMT which was group III nitride semiconductor device 4 obtainedin this way, GaN layer 26 and Al_(0.2)Ga_(0.8)N layer 27 were formed asat least one group III nitride layer 20 on group III nitride film 13 ofgroup III nitride composite substrate 1, and source electrode 60, drainelectrode 70, and gate electrode 80 were disposed on Al_(0.2)Ga_(0.8)Nlayer 27 so that the electrodes were separated from each other and gateelectrode 80 was located between source electrode 60 and drain electrode70.

The leakage gate current density of the obtained HEMT was inspected.Specifically, the produced devices were classified into non-defectivedevices meeting a standard that the leakage gate current density was1×10⁻⁶ A/cm² or less when being applied with a gate voltage of 5 V, anddefective devices failing to meet this standard, and the percentage ofthe ratio determined by dividing the non-defective devices by the sum ofthe non-defective devices and the defective devices was defined as ayield.

Mean value m_(III-N) and standard deviation s_(III-N) of the RMS of themain surface of the group III nitride film in the group III nitridecomposite substrate, mean value ms and standard deviation s_(S) of theRMS of the main surface of the support substrate, and the yield of groupIII nitride semiconductor devices are summarized in Table 10.

TABLE 10 Reference Example I-H I-H1 I-H2 I-H3 I-H4 I-H5 I-H6 I-H7 I-H8I-H9 I-H10 I-H11 I-H12 mean value m_(III-N) of RMS 0.1 0.5 1 2 3 2 2 2 22 2 2 of main surface of group III nitride film (nm) standard deviations_(III-N) of 0.03 0.1 0.2 0.4 0.4 0.5 0.4 0.4 0.4 0.4 0.4 0.4 RMS ofmain surface of group III nitride film (nm) mean value m_(S) of RMS of 22 2 2 2 2 0.3 2 5 10 13 10 main surface of support substrate (nm)standard deviation s_(S) of 0.2 0.2 0.2 0.2 0.2 0.2 0.1 0.4 1.4 3 3 4RMS of main surface of support substrate (nm) device yield (%) 72 69 6560 51 52 68 67 64 57 51 52

Referring to Table 10, group III nitride semiconductor devicesmanufactured with a high yield were those using a group III nitridecomposite substrate in which the main surface of the group III nitridefilm had a mean value m_(III-N) of the RMS of 0.1 nm or more and 2 nm orless and a standard deviation s_(III-N) of the RMS of 0.4 nm or less,and the main surface of the support substrate had a mean value ms of theRMS of 0.3 nm or more and 10 nm or less and a standard deviation s_(S)of the RMS of 3 nm or less.

Reference Example I-I

1. Fabrication of Group III Nitride Composite Substrate

Referring to FIG. 14 (A) to (C), joined substrate 1L was produced in asimilar manner to Reference Example 1-A, except that: an Al₂O₃—SiO₂composite oxide substrate (with respect to the whole substrate, Al₂O₃was 88% by mass and SiO₂ was 12% by mass) having a diameter of 100 mmwas prepared for use as support substrate 11; a GaN crystal substratehaving a diameter of 100 mm and a thickness of 400 μm was prepared foruse as group III nitride film donor substrate 13D; on respective mainsurfaces of support substrate 11 and group III nitride film donorsubstrate 13D, an SiO₂ film was grown to a thickness of 500 nm by thePE-CVD method, and thereafter CMP was performed using a slurry having apH of 9 and containing colloidal silica having an average grain size of20 nm, to thereby form joint films 12 a, 12 b having a thickness of 250nm and having a main surface planarized to have an RMS roughness of 0.15nm or less; and scrub cleaning was performed with pure water and asponge made of PVA (polyvinyl alcohol), as the cleaning with pure water.

Referring to FIG. 14 (D), grinding and polishing were performed on themain surface of group III nitride film donor substrate 13D in joinedsubstrate 1L that was opposite to the bonded main surface thereof. Forgrinding, a vitrified grinding wheel containing diamond abrasive grainshaving an average grain size of 25 gtm to 35 μm was used. For polishing,mechanical polishing was performed in a stepwise manner using slurriescontaining diamond abrasive grains having an average grain size of 3gim, 2 μm, and 0.25 μm, respectively. This polishing was performed, inorder to reduce the thickness variation and the off-angle variation, insuch a manner that the warp of the substrate was corrected in advance bymechanical pressurization and the substrate in this state was bonded toa holding plate. After polishing, dry etching was performed by means ofchlorine gas plasma which was generated by the ICP-RIE (InductivelyCoupled Plasma Reactive Ion Etching) method. It should be noted,Reference Example I-18 used, instead of the above-described grinding,rough polishing performed by means of diamond abrasive grains having anaverage grain size of 9 μm. In this way, group III nitride compositesubstrate 1 including group III nitride film 13 having a thickness of150 μm was obtained.

2. Fabrication of Group III Nitride Semiconductor Device

Referring to FIG. 15, group III nitride semiconductor device 4 wasproduced in a similar manner to Reference Example I-A, except that:group III nitride composite substrate 1 of the present Reference Examplewas used; support substrate 11 and joint film 12 were removed fromlaminated substrate 3 by grinding with a vitrified grinding wheelcontaining diamond abrasive grains having an average grain size of 35 μmto 45 μm, except that, for Reference Example 1-19, they were removed bypolishing with diamond abrasive grains having an average grain size of15 μm; in order to remove residue of the joint film and the supportsubstrate after the grinding and/or the polishing, etching cleaning wasperformed with hydrofluoric acid.

In a similar manner to Reference Example I-A, the ratio s_(t)/m_(t) ofstandard deviation s_(t) of the thickness of group III nitride film 13,to mean value m_(t) of the thickness thereof, and the ratio s_(o)/m_(o)of standard deviation s_(o) of an absolute value of an off angle betweenthe main surface of the group III nitride film and the (0001) plane, tomean value m_(o) of the absolute value of the off angle thereof werecalculated for group III nitride composite substrate 1. The results aresummarized in Table 11. The yield of group III nitride semiconductordevices 4 was also calculated in a similar manner to Reference ExampleI-A. The results are summarized in Table 11.

TABLE 11 Reference Example I-I I-I1 I-I2 I-I3 I-I4 I-I5 I-I6 I-I7 I-I8I-I9 ratio s_(t)/m_(t) 0.001 0.002 0.05 0.15 0.2 0.25 0.2 0.05 0.05ratio s_(o)/m_(o) 0.005 0.008 0.2 0.5 0.6 0.6 0.7 0.2 0.2 device yield(%) 76 76 72 61 56 34 31 71 73

Referring to Table 11, group III nitride semiconductor devicesmanufactured with a high yield were those using a group III nitridecomposite substrate including a group III nitride film where the ratios_(t)/m_(t) of standard deviation s_(t) of the thickness of the groupIII nitride film, to mean value m_(t) of the thickness thereof was 0.001or more and 0.2 or less, and the ratio s_(o)/m_(o) of standard deviations_(o) of an absolute value of an off angle between the main surface ofthe group III nitride film and the (0001) plane, to mean value m_(o) ofthe absolute value of the off angle thereof was 0.005 or more and 0.6 orless. It should be noted that, as to Example I-I1, sophisticated controlwas necessary for the grinding and polishing in order to reduce thethickness variation and the off-angle variation, and thus the grindingand polishing took a long time.

Reference Example II-A to Reference Example II-H according to ReferenceInvention II will be illustrated below.

Reference Example II-A

1. Fabrication of Group III Nitride Composite Substrate

Referring to FIG. 12 (A), an Al₂O₃—SiO₂ composite oxide substrate (withrespect to the whole substrate, Al₂O₃ was 85% by mass and SiO₂ was 15%by mass) having a diameter of 75 mm and a thickness of 300 μm wasprepared for use as support substrate 11. Support substrate 11 had athermal conductivity of 10 W·m⁻¹·K⁻¹ and a Young's modulus of 250 GPa.Main surfaces 11 m, 11 n of the two opposite sides respectively ofsupport substrate 11 were subjected to rough polishing with acopper-based surface plate, intermediate polishing with a tin surfaceplate, and finish polishing with a nonwoven polishing pad in whichgrating grooves were formed, for which a diamond slurry was used as anabrasive. The finish polishing was performed under the condition thataction coefficient FE was 4×10⁻¹⁷ m²/s or more and 1×10⁻¹⁶ m²/s or less.

Next, on finish-polished main surface 11 m of support substrate 11, anSiO₂ film of 800 nm in thickness was grown by the PE-CVD (PlasmaEnhanced-Chemical Vapor Deposition) method, annealed in a nitrogenatmosphere at 800° C. for an hour, and thereafter subjected to CMP(Chemical Mechanical Polishing) using a slurry with a pH of 10containing colloidal silica abrasive grains having an average grain sizeof 40 nm, to thereby form joint film 12 a of 400 nm in thickness havingits main surface 12 am mirror-finished so that the RMS (root mean squareroughness) was 0.3 nm or less. Subsequently, in order to remove thecolloidal silica abrasive grains used for the CMP, non-abrasivepolishing cleaning with a KOH aqueous solution, polishing cleaning withpure water, and megasonic cleaning (cleaning with ultrasonic waves in amegasonic frequency range of 500 kHz to 5 MHz) with pure water wereperformed.

Referring also to FIG. 12 (B), a GaN crystal body having a diameter of75 mm and a thickness of 8 mm was prepared for use as group III nitridefilm donor substrate 13D. A surface-to-be-bonded of group III nitridefilm donor substrate 13D was subjected to mechanical polishing and CMPso that it was planarized to have an RMS of 2 nm or less. After this, anSiO₂ film of 800 nm in thickness was grown on this substrate by thePE-CVD method, annealed in a nitrogen atmosphere at 800° C. for an hour,and thereafter subjected to CMP using a slurry with a pH of 10containing colloidal silica abrasive grains having an average grain sizeof 40 nm, to thereby form joint film 12 b of 500 nm in thickness havingits main surface 12 bn mirror-finished so that the RMS was 0.3 nm orless. Subsequently, in order to remove the colloidal silica abrasivegrains used for the CMP, non-abrasive polishing cleaning with a KOHaqueous solution, polishing cleaning with pure water, and megasoniccleaning (cleaning with ultrasonic waves in a megasonic frequency rangeof 500 kHz to 5 MHz) with pure water were performed. Here, group IIInitride film donor substrate 13D was produced through growth by the HVPEmethod using a GaAs substrate as a base substrate. Group III nitridefilm donor substrate 13D had its electrical conductivity of n-type, itsdislocation density of 1×10⁸ cm⁻², and its carrier concentration of1×10¹⁷.

Referring next to FIG. 12 (C), main surface 12 am of joint film 12 a andmain surface 12 bn of joint film 12 b were bonded together to therebyproduce joined substrate 1L in which support substrate 11 and group IIInitride film 13 were bonded to each other with joint film 12 interposedtherebetween. After they were bonded together, joined substrate 1L wasannealed by being heated in a nitrogen gas atmosphere to 800° C., tothereby increase the joint strength.

Referring next to FIG. 12 (D), group III nitride film donor substrate13D in joined substrate 1L was cut, with a wire saw, along a planelocated inwardly at a depth of 150 μm from the bonded surface wheregroup III nitride film donor substrate 13D was bonded to joint film 12,to thereby produce group III nitride composite substrate 1 in whichsupport substrate 11 and the GaN film which was group III nitride film13 were bonded together with joint film 12 interposed therebetween. Asthe wire, a fixed-abrasive wire with a diameter of 180 μm on whichdiamond abrasive grains were electrodeposited was used. As for thecutting method, in order to reduce the cut resistance and enhance thethickness precision and the flatness, the method was used that causedthe wire to swing and caused group III nitride film donor substrate 13Dto vibrate in synchronization therewith. The resistance coefficient forcutting with the wire saw was set to 4200 N. After cutting, group IIInitride film 13 of group LlI nitride composite substrate 1 was subjectedto mechanical polishing and CMP. In order to have a uniform thickness ofgroup III nitride film 13, the composite substrate was mounted on a CMPapparatus in the following way. The shape of the substrate was correctedin advance by vacuum chuck suction, and thereafter the compositesubstrate was suction-fixed onto the apparatus.

Next, Group III-nitride-film 13-side main surface 13 m of group IIInitride composite substrate 1 thus obtained was subjected to roughpolishing with a copper-based surface plate and intermediate polishingwith a tin surface plate, using a diamond slurry as an abrasive, andfurther subjected to finish polishing with a nonwoven polishing padusing, as an abrasive, a slurry with a pH of 11 containing colloidalsilica abrasive grains. The finish polishing was performed under thecondition that action coefficient FE was 6×10⁻¹⁴ m²/s. After the finishpolishing, group III nitride film 13 had a thickness of 110 μm.

For group III nitride composite substrate 1 after this finish polishing,mean value ms of the RMS of support-substrate 11-side main surface 11 n,and the ratio s_(S)/m_(S) of standard deviation s_(S) of the RMS to meanvalue ms of the RMS of support-substrate 11-side main surface 11 n aresummarized in Table 12.

Here, mean value ms of the RMS and the ratio s_(S)/m_(S) of standarddeviation s_(S) of the RMS to mean value ms of the RMS were calculatedfrom the RMS of support-substrate 11-side main surface 11 n at the 13measurement points P on support-substrate 11-side main surface 11 nshown in FIG. 7, constituted of: one central point P_(C); four outerpoints P_(O) located respectively in the four directions with respect tocentral point P_(C) that are orthogonal to each other, the outer pointseach being located at 5 mm inward from the outer edge of the substrate;and eight middle points P_(M) including four middle points each betweenthe one central point P_(C) and one of the four outer points P_(O) andfour middle points each between two of the four outer points P_(O).

2. Fabrication of Group III Nitride Semiconductor Device

Referring to FIG. 15 (A), on group III-nitride-film 13-side main surface13 m of group III nitride composite substrate 1, group III nitride layer20 was formed by the MOVPE method. Specifically, on group III nitridefilm 13, n-GaN layer 21 having a thickness of 5 μm,n-In_(0.05)Ga_(0.95)N layer 22 having a thickness of 50 nm, active layer23 having a multiple quantum well structure of three cycles constitutedof an In_(0.14)Ga_(0.86)N well layer having a thickness of 3 nm and aGaN barrier layer having a thickness of 15 nm, p-Al_(0.09)Ga_(0.91)Nlayer 24 having a thickness of 20 nm, and p-GaN layer 25 having athickness of 150 nm were successively epitaxially grown to therebyproduce laminated group III nitride composite substrate 2. After this,it was annealed by an RTA (Rapid Thermal Annealing) apparatus andaccordingly activated.

Referring to FIG. 15 (B), on p-GaN layer 25, which was the topmost layerin group III nitride layer 20 of laminated group III nitride compositesubstrate 2, an Ni layer having a thickness of 4 nm and an Au layerhaving a thickness of 200 nm were successively formed by the EB(Electron Beam) deposition method, and annealed into an alloy, tothereby form first electrode 30. On first electrode 30, a Ti layerhaving a thickness of 200 nm, a Pt layer having a thickness of 100 nm,and an Au layer having a thickness of 1000 nm were successively formedby the EB deposition method to thereby form pad electrode 33.

A CuW substrate was prepared for use as device support substrate 40. Ondevice support substrate 40, a Ti layer having a thickness of 200 nm, aPt layer having a thickness of 100 nm, and an Au layer having athickness of 1000 nm were successively formed by the EB depositionmethod to thereby form pad electrode 43. On pad electrode 43, an AuSnsolder film was formed as joint metal film 44.

Subsequently, joint metal film 44 was bonded to pad electrode 33 tothereby produce laminated substrate 3.

Referring to FIG. 15 (C), from laminated substrate 3, support substrate11 and joint film 12 in group III nitride composite substrate 1 wereetched away by means of hydrofluoric acid.

Referring to FIG. 15 (D), on group III nitride film 13 having beenexposed by removal of support substrate 11 and joint film 12 fromlaminated substrate 3, a Ti layer having a thickness of 20 nm, an Allayer having a thickness of 200 nm, and an Au layer having a thicknessof 300 nm were successively formed by the EB deposition method, and thenannealed to form second electrode 50. On device support substrate 40, aTi layer having a thickness of 20 nm and an Au layer having a thicknessof 300 nm were successively formed by the EB deposition method, andannealed to thereby form device support substrate electrode 45. In thisway, group III nitride semiconductor device 4 was obtained.

For group III nitride semiconductor device 4 thus obtained, its opticaloutput was measured by means of an integrating sphere under thecondition that injected current was 4 A. The optical output of thelight-emitting device was measured in the following way. Specifically,into the light-emitting device mounted in the integrating sphere,predetermined current was injected, and the optical output was measuredby a detector receiving the light collected from the light-emittingdevice. The resultant group III nitride semiconductor devices wereclassified into non-defective devices meeting a standard that theoptical output was 2 W or more and defective devices failing to meetthis standard, and the percentage of the ratio determined by dividingthe non-defective devices by the sum of the non-defective devices andthe defective devices was defined as a yield. The yield of the group IIInitride semiconductor devices is summarized in Table 12.

TABLE 12 Reference Example II-A II-A1 II-A2 II-A3 II-A4 II-A5 II-A6II-A7 II-A8 II-A9 II-A10 II-A11 II-A12 II-A13 m_(s) (nm) 0.3 0.5 1 2 510 20 0.3 20 30 35 20 10 ratio s_(s)/m_(s) 0.005 0.01 0.05 0.1 0.2 0.30.4 0.4 0.005 0.4 0.005 0.5 0.65 device yield (%) 77 77 73 71 67 63 5663 58 31 32 35 37

Referring to Table 12, group III nitride semiconductor devicesmanufactured with a high yield were those fabricated using a group IIInitride composite substrate with a diameter of 75 mm (this value meets75 mm or more) including a support substrate and a group III nitridefilm with a thickness of 110 μm (this value meets 10 μm or more and 250μm or less), where mean value ms of the RMS of thesupport-substrate-side main surface was 0.3 nm or more and 20 nm orless, and ratio s_(S)/m_(S) of standard deviation s_(S) of the RMS tomean value ms of the RMS of the support-substrate-side main surface was0.005 or more and 0.4 or less.

Reference Example II-B

Referring to FIGS. 12 and 15, group III nitride composite substrate 1and group III nitride semiconductor device 4 were produced in a similarmanner to Reference Example II-A, except that: a mullite-YSZ substrate(with respect to the whole substrate, mullite was 70% by mass and YSZwas 30% by mass, with respect to mullite, Al₂O₃ was 60% by mole and SiO₂was 40% by mole and, with respect to YSZ, ZrO₂ was 90% by mole and Y₂O₃was 10% by mole) was used as support substrate 11; different diametersin a range of 75 mm to 150 mm were used and support-substrate 11-sidemain surface 11 n was finish-polished under the condition that actioncoefficient FE was 6.2×10⁻¹⁷ m²/s; and group III nitride compositesubstrates 1 were fabricated in which respective group III nitride films13 after finish-polished had different thicknesses in a range of 10 μmto 250 μm. Support substrate 11 had a thermal conductivity of 30W·m⁻¹·K⁻¹ and a Young's modulus of 150 GPa.

In a similar manner to Reference Example II-A, for group III nitridecomposite substrate 1, mean value ms of the RMS of support-substrate11-side main surface 11 n and the ratio s_(S)/m_(S) of standarddeviation s_(S) of the RMS to mean value ms of the RMS ofsupport-substrate 11-side main surface 11 n were calculated andsummarized in Table 13. In a similar manner to Reference Example 11-A,the yield of group III nitride semiconductor devices 4 was alsocalculated and summarized in Table 13.

TABLE 13 Reference Example II-B II-B1 II-B2 II-B3 II-B4 II-B5 II-B6II-B7 II-B8 II-B9 II-B10 II-B11 II-B12 diameter D (mm) 75 75 75 75 75100 100 100 100 125 150 150 thickness of 10 30 100 200 250 10 110 180250 120 130 250 group III nitride film (μm) m_(s) (nm) 6 6 6 6 6 6 6 6 66 6 6 ratio s_(s)/m_(s) 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.23 0.230.23 0.23 0.23 device yield (%) 58 62 69 72 65 59 74 73 66 75 74 62

Referring to Table 13, group III nitride semiconductor devicesmanufactured with a high yield were those fabricated using a group IIInitride composite substrate with a diameter of 75 mm to 150 mm includinga group III nitride film with a thickness of 10 μm to 250 μm, where meanvalue ms of the RMS of the support-substrate-side main surface was 6 nm(this value meets 0.3 nm or more and 20 nm or less), and the ratios_(S)/m_(S) of standard deviation s_(S) of the RMS to mean value ms ofthe RMS of the support-substrate-side main surface was 0.23 (this valuemeets 0.005 or more and 0.4 or less).

Reference Example II-C

As substrates for semiconductor devices, a group III nitridefree-standing substrate (hereinafter also referred to as FS substrate),a group III nitride composite substrate (hereinafter also referred to asBP substrate) produced by the ion implantation method, and a group IIInitride composite substrate (hereinafter also referred to as BSsubstrate) produced in accordance with Reference Embodiment II-4 ofReference Invention III were prepared.

The FS substrate was prepared to have the diameter and the thicknessshown in Table 4, by cutting a GaN crystal body having a predetermineddiameter with a wire saw and polishing it.

The BP substrate was prepared to have the diameter and the thickness ofits group III nitride film shown in Table 14. Specifically, as shown inFIG. 17 (B), hydrogen ions were implanted from main surface 13 n of theGaN crystal body having a predetermined diameter, namely group IIInitride film donor substrate 13D, to the position of a predetermineddepth located inwardly from the main surface, to thereby form an ionimplantation region 13 i. After this, as shown in FIG. 17 (C), supportsubstrate 11 and the ion implantation region 13 i side of group IIInitride film donor substrate 13D were bonded together with joint film 12interposed therebetween. After this, as shown in FIG. 17 (D), annealingwas done at 700° C. to separate group III nitride film donor substrate13D along its ion implantation region 13 i. Here, an Al₂O₃—SiO₂composite oxide substrate (with respect to the whole substrate, Al₂O₃was 85% by mass and SiO₂ was 15% by mass) was used as support substrate11.

The BS substrate was prepared to have the diameter and the thickness ofits group III nitride film as shown in Table 14, in a similar manner toReference Example II-B except that an Al₂O₃—SiO₂ composite oxidesubstrate (with respect to the whole substrate, Al₂O₃ was 85% by massand SiO₂ was 15% by mass) was used as the support substrate.

Group III nitride composite substrates 1 and group III nitridesemiconductor devices 4 were fabricated in a similar manner to ReferenceExample II-B, except that the above-described FS substrate, BPsubstrate, and BS substrate were used, finish polishing ofsupport-substrate 11-side main surface 11 n was performed under thecondition that action coefficient FE was 4.0×10⁻¹⁷ m²/s.

In a similar manner to Reference Example II-A, for the BP substrate andthe BS substrate each serving as group III nitride composite substrate1, warp W, ratio W/D, mean value ms of the RMS of support-substrate11-side main surface 11 n, and the ratio s_(S)/m_(S) of standarddeviation s_(S) of the RMS to mean value ms of the RMS ofsupport-substrate 11-side main surface 11 n were calculated andsummarized in Table 14. For the FS substrate as well, warp W, ratio W/D,mean value ms of the RMS of the rear-side main surface, and the ratios_(S)/m_(S) of standard deviation s_(S) of the RMS to mean value ms ofthe RMS of rear-side main surface were calculated and summarized inTable 14. Further, in a similar manner to Reference Example II-A, theyield of group III nitride semiconductor devices 4 was calculated andsummarized in Table 14.

TABLE 14 FS substrate BP substrate BS substrate Reference Example II-CII-C1 II-C2 II-C3 II-C4 II-C5 II-C6 II-C7 II-C8 II-C9 II-C10 II-C11II-C12 II-C13 II-C14 diameter D (mm) 50 75 75 100 100 100 125 75 100 12575 100 125 150 thickness of group 280 250 380 250 450 450 500 0.5 0.50.5 140 140 140 140 III nitride film (μm) warp W (μm) 40 120 42 170 2525 40 20 25 30 25 25 40 45 ratio W/D (×10⁻⁴) 8 16 5.6 17 2.5 2.5 3.2 2.72.5 2.4 3.3 2.5 3.2 3 m_(s) (nm) 4 4 4 4 4 30 4 4 4 4 4 4 4 4 ratios_(s)/m_(s) 0.15 0.15 0.15 0.15 0.15 0.5 0.15 0.15 0.15 0.15 0.15 0.150.15 0.15 device yield (%) 52 crack 54 crack 56 18 52 0 0 0 72 73 70 69

Referring to Table 14, as to group III nitride semiconductor devicesfabricated using FS substrates with a diameter of 50 mm to 125 mm and athickness of 250 μm to 500 μm, the group III nitride semiconductordevices with a relatively large diameter and a relatively smallthickness had a large warp and were likely to crack, and the yields ofthem were all less than 60%.

Regarding group III nitride semiconductor devices fabricated using BPsubstrates with a diameter of 75 mm to 125 mm and a thickness of 0.5 μmof the group III nitride film, the devices did not exhibit excellentdevice characteristics due to the relatively small thickness of thegroup III nitride film, resulting in lower yields.

In contrast, the yields of the group III nitride semiconductor devicesfabricated using BS substrates were higher, namely 69% or more. The BSsubstrate has a larger thickness of the group III nitride film to allowelectric current to be sufficiently distributed, as compared with the BPsubstrate. In contrast to the BP substrate, the BS substrate has nodeterioration, due to ion implantation, of the crystal quality of thegroup III nitride film. Therefore, the semiconductor devices fabricatedusing the BS substrate exhibited excellent characteristics.

Reference Example II-D

Referring to FIGS. 12 and 15, group III nitride composite substrate 1and group III nitride semiconductor device 4 were fabricated in asimilar manner to Reference Example II-A, except that: an Al₂O₃—SiO₂composite oxide substrate (with respect to the whole substrate, Al₂O₃was 82% by mass and SiO₂ was 18% by mass) was used as support substrate11; different diameters in a range of 75 mm to 150 mm were used, an SiO₂film serving as joint film 12 was grown by the AP-CVD (AtmosphericPressure-Chemical Vapor Deposition) method, and support-substrate11-side main surface 11 n was subjected to finish polishing under thecondition that action coefficient FE was 8.5×10⁻¹⁷ m²/s; and respectivefinish-polished group III nitride films 13 had different thicknesses ina range of 110 μm to 130 μm in produced group III nitride compositesubstrates 1. Support substrate 11 had a thermal conductivity of 5W·m⁻¹·K⁻¹ and a Young's modulus of 230 GPa.

In a similar manner to Reference Example II-A, for group III nitridecomposite substrate 1, mean value ms of the RMS of support-substrate11-side main surface 11 n, the ratio s_(S)/m_(S) of standard deviations_(S) of the RMS to mean value ms of the RMS of support-substrate11-side main surface 11 n, warp W of support-substrate 11-side mainsurface 11 n, and the ratio W/D of warp W of support-substrate 11-sidemain surface 11 n to diameter D were calculated and summarized in Table15. In a similar manner to Reference Example II-A, the yield of groupIII nitride semiconductor devices 4 was also calculated and summarizedin Table 15.

TABLE 15 Reference Example II-D II-D1 II-D2 II-D3 II-D4 II-D5 II-D6II-D7 II-D8 II-D9 II-D10 II-D11 II-D12 II-D13 II-D14 II-D15 II-D16diameter D (mm) 100 100 100 100 100 100 100 100 100 100 75 150 150 150150 150 thickness of 110 110 110 110 110 110 110 110 110 110 130 120 120120 120 120 group III nitride film (μm) ratio W/D −10 −7 −4 −2.5 −1 1.53 5 8 10 2 −11 −7 −2.5 8 10 (×10⁻⁴) m_(s) (nm) 3 3 3 3 3 3 3 3 3 3 3 3 33 3 3 ratio s_(s)/m_(s) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.10.1 0.1 0.1 0.1 device yield (%) 63 70 72 74 75 76 73 72 69 64 74 58 6672 65 59

Referring to Table 15, group III nitride semiconductor devicesmanufactured with a high yield were those fabricated using a group IIInitride composite substrate with a diameter of 75 mm to 150 mm (thisvalue meets 75 mm or more) including a group III nitride film with athickness of 110 μm to 130 μm (this value meets 10 μm or more and 250 μmor less), where mean value ms of the RMS of the support-substrate-sidemain surface was 3 nm (this value meets 0.3 nm or more and 20 nm orless), the ratio s_(S)/m_(S) of standard deviation s_(S) of the RMS tomean value ms of the RMS of the support-substrate-side main surface was0.1 (this value meets 0.005 or more and 0.4 or less), and the ratio W/Dof warp W of support-substrate 11-side main surface 111 n to diameter Dwas −7×10^(0.4) or more and 8×10⁴ or less.

Reference Example II-E

Referring to FIGS. 12 and 15, group III nitride composite substrate 1and group III nitride semiconductor device 4 were produced in a similarmanner to Reference Example II-A, except that: a mullite substrate (withrespect to the whole mullite substrate, Al₂O₃ was 60% by mole and SiO₂was 40% by mole) was used as support substrate 11; a GaN substrate withhigh electrical conductivity doped with O (oxygen) atoms and Si(silicon) atoms, having no dislocation concentrated region, having aconstant dislocation density of 5×10⁶ cm⁻², and a carrier concentrationof 2×10¹⁸ cm⁻³ was used as group III nitride film donor substrate 13D;support-substrate 11-side main surface 11 n was subjected to finishpolishing under the condition that action coefficient FE was 8.3×10⁻¹⁷m²/s; group III nitride film donor substrate 13D was cut by electricaldischarge wire cutting; and group III-nitride-film 13-side main surface13 m was subjected to finish polishing with a slurry containingcolloidal silica with a grain size of 20 nm to 400 nm under thecondition that action coefficient FE was 4×10⁻¹⁴ or more and 1×10⁻¹³ orless. Support substrate 11 had a thermal conductivity of 3 W·m⁻¹·K⁻¹ anda Young's modulus of 200 GPa.

In a similar manner to Reference Example II-A, for finish-polished groupIII nitride composite substrate 1, mean value ms of the RMS ofsupport-substrate 11-side main surface 11 n, and the ratio s_(S)/m_(S)of standard deviation s_(S) of the RMS to mean value ms of the RMS ofsupport-substrate 11-side main surface 11 n were calculated andsummarized in Table 16.

Further, for finish-polished group III nitride composite substrate 1,mean value m_(III-N) of the RMS of group III-nitride-film 13-side mainsurface 13 m, and the ratio s_(III-N)/m_(III-N) of standard deviations_(III-N) of the RMS to mean value m_(III-N) of the RMS of groupIII-nitride-film 13-side main surface 13 m are summarized in Table 16.Here, mean value m_(III-N) of the RMS and the ratio s_(III-N)/m_(III-N)of standard deviation s_(III-N) of the RMS to mean value m_(III-N) ofthe RMS were calculated from the RMS of group III-nitride-film 13-sidemain surface 13 m at 13 measurement points P on group III-nitride-film13-side main surface 13 m shown in FIG. 7 constituted of: one centralpoint P_(C); four outer points P_(O) located respectively in the fourdirections with respect to central point P_(C) that are orthogonal toeach other, the outer points each being located at 5 mm inward from theouter edge; and eight middle points P_(M) including four middle pointseach between the one central point P_(C) and one of the four outerpoints P_(O) and four middle points each between two of the four outerpoints P_(O).

Further, in a similar manner to Reference Example II-A, the yield ofgroup III nitride semiconductor devices 4 was calculated and summarizedin Table 16.

TABLE 16 Reference Example II-E II-E1 II-E2 II-E3 II-E4 II-E5 II-E6II-E7 II-E8 II-E9 II-E10 II-E11 II-E12 II-E13 m_(III-N) (nm) 0.15 0.40.8 1 1.6 2 3 0.15 3 4 4.5 3 2.5 ratio 0.008 0.02 0.05 0.1 0.2 0.4 0.50.5 0.008 0.5 0.008 0.7 0.8 s_(III-N)/m_(III-N) m_(s) (nm) 2 2 2 2 2 2 22 2 2 2 2 2 ratio s_(s)/m_(s) 0.12 0.12 0.12 0.12 0.12 0.12 0.12 0.120.12 0.12 0.12 0.12 0.12 device yield (%) 80 80 75 72 68 63 59 62 60 5251 53 54

Referring to Table 16, group III nitride semiconductor devicesmanufactured with a high yield were those fabricated using a group IIInitride composite substrate with a diameter of 75 mm (this value meets75 mm or more) including a group III nitride film with a thickness of110 μm (this value meets 10 μm or more and 250 μm or less), where meanvalue ms of the RMS of the support-substrate-side main surface was 2 nm(this value meets 0.3 nm or more and 20 nm or less), the ratios_(S)/m_(S) of standard deviation s_(S) of the RMS to mean value ms ofthe RMS of the support-substrate-side main surface was 0.12 (this valuemeets 0.005 or more and 0.4 or less), mean value m_(III-N) of the RMS ofthe group III-nitride-film-side main surface was 0.15 nm or more and 3nm or less, and the ratio s_(III-N)/m_(III-N) of standard deviations_(III-N) of the RMS to mean value m_(III-N) of the RMS of the groupIII-nitride-film-side main surface was 0.008 or more and 0.5 or less.

Reference Example II-F

1. Fabrication of Group III Nitride Composite Substrate

Referring to FIG. 12, group III nitride composite substrates 1 werefabricated in a similar manner to Reference Example II-A, except that:an Al₂O₃—SiO₂ composite oxide substrate, a mullite substrate (withrespect to the whole substrate, Al₂O₃ was 60% by mole and SiO₂ was 40%by mole), and a mullite-YSZ substrate (with respect to the wholesubstrate, mullite was 70% by mass and YSZ was 30% by mass, with respectto mullite, Al₂O₃ was 60% by mole and SiO₂ was 40% by mole and, withrespect to YSZ, ZrO₂ was 90% by mole and Y₂O₃ was 10% by mole) were eachused as support substrate 11; a semi-insulating GaN substrate doped withFe (iron) atoms and having an electrical resistance (also referred to asspecific resistance) of 2×10⁵ Ωcm was used as group III nitride filmdonor substrate 13D, the group III nitride film donor substrate was cutwith a wire saw using loose abrasive grains; support-substrate 11-sidemain surface 11 n was subjected to finish polishing under the conditionthat action coefficient FE was 8.7×10⁻¹⁷ m²/s; and the group III nitridecomposite substrates were fabricated with different diameters in a rangeof 75 mm to 150 mm.

2. Fabrication of Group III Nitride Semiconductor Device

Referring to FIG. 10, in the present reference example, an HEMT wasfabricated as group III nitride semiconductor device 4. On main surface13 m of group III nitride film 13 of group III nitride compositesubstrate 1, GaN layer 26 having a thickness of 1.5 μm andAl_(0.2)Ga_(0.8)N layer 27 having a thickness of 30 nm were successivelyepitaxially grown by the MOVPE method to form group III nitride layer20, to thereby produce laminated group III nitride composite substrate2.

Next, on Al_(0.2)Ga_(0.8)N layer 27, a source electrode 60 and a drainelectrode 70 were fabricated by photolithography, EB deposition, andlift-off. In these electrodes, a Ti layer having a thickness of 20 nm,an Al layer having a thickness of 100 nm, a Ti layer having a thicknessof 20 nm, and an Au layer having a thickness of 300 nm were successivelyformed, lifted off, and annealed for 1 minute at 600° C. into an alloy.

Next, in a similar step to the step of forming source electrode 60 anddrain electrode 70, a gate electrode 80 was formed. For formation ofgate electrode 80, an Ni layer having a thickness of 50 nm and an Aulayer having a thickness of 500 nm were successively formed. The gatelength was set to 2 μm.

In the HEMT which was group III nitride semiconductor device 4 obtainedin this way, GaN layer 26 and Al_(0.2)Ga_(0.8)N layer 27 were formed asat least one group III nitride layer 20 on group III nitride film 13 ofgroup III nitride composite substrate 1, and source electrode 60, drainelectrode 70, and gate electrode 80 were disposed on Al_(0.2)Ga_(0.8)Nlayer 27 so that the electrodes were separated from each other and gateelectrode 80 was located between source electrode 60 and drain electrode70.

In a similar manner to Reference Example II-A, for group III nitridecomposite substrate 1, mean value ms of the RMS of support-substrate11-side main surface 11 n and the ratio s_(S)/m_(S) of standarddeviation s_(S) of the RMS to mean value ms of the RMS ofsupport-substrate 11-side main surface 11 n were calculated andsummarized in Table 17. Further, thermal expansion coefficient α_(S) ofsupport substrate 11 and thermal expansion coefficient α_(III-N) ofgroup III nitride film 13 were measured with a thermomechanical analysisapparatus, the ratio α_(III-N)/α_(S) of thermal expansion coefficientα_(III-N) of the group III nitride film to thermal expansion coefficientα_(S) of the support substrate was calculated, and the results weresummarized in Table 17. Further, thickness t_(S) of support substrate 11and thickness t_(III-N) of group III nitride film 13 were measured witha digital indicator, the ratio t_(III-N)/t_(S) of thickness t_(III-N) ofgroup III nitride film 13 to thickness t_(S) of support substrate 11 wascalculated, and the results were summarized in Table 17.

In the following way, the yield of group III nitride semiconductordevices 4 was also calculated. Specifically, HEMTs which were group IIInitride semiconductor devices 4 were classified into non-defectivedevices meeting a standard that the leakage gate current density was1×10⁻⁶ A/cm² or less when being applied with a gate voltage of 5 V, anddefective devices failing to meet this standard, and the percentage ofthe ratio determined by dividing the non-defective devices by the sum ofthe non-defective devices and the defective devices was defined as ayield.

TABLE 17 Reference Example II-F II-F1 II-F2 II-F3 II-F4 II-F5 II-F6II-F7 II-F8 II-F9 II-F10 II-F11 II-F12 II-F13 II-F14 diameter D (mm) 7575 75 75 75 100 100 100 100 100 100 100 150 150 type of supportAl₂O₃—SiO₂ mullite-YSZ Al₂O₃—SiO₂ mullite Al₂O₃—SiO₂ substrate ratioα_(III-N)/α_(S) 1 1 1 1 1 1 1 1 1.25 0.9 0.75 1.2 1.2 1 ratiot_(III-N)/t_(S) 0.02 0.1 0.3 0.6 1 0.4 0.4 1 0.4 0.4 0.4 0.3 0.2 0.2m_(s) (nm) 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 1.5 ratios_(s)/m_(s) 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1 0.1device yield (%) 62 65 75 66 64 76 72 63 63 70 62 64 63 69

Referring to Table 17, group III nitride semiconductor devicesfabricated with a high yield were those using group III nitridecomposite substrate 1 in which, regardless of whether the Al₂O₃—SiO₂composite oxide substrate, the mullite substrate, or the mullite-YSZsubstrate was used as support substrate 11, the thickness of the groupIII nitride film of the composite substrate was 110 μm (this value meets10 μm to 250 μm), the diameter of the composite substrate was 75 mm to150 mm, mean value ms of the RMS of the support-substrate-side mainsurface was 1.5 nm (this value meets 0.3 nm or more and 20 nm or less),and the ratio s_(S)/m_(S) of standard deviation s_(S) of the RMS to meanvalue ms of the RMS of the support-substrate-side main surface was 0.1(this value meets 0.005 or more and 4.0 or less). Group III nitridesemiconductor devices fabricated with a high yield were also those inwhich the ratio α_(III-N)/α_(S) of thermal expansion coefficientα_(III-N) of group III nitride film 13 to thermal expansion coefficientas of support substrate 11 was 0.75 or more and 1.25 or less, and theratio t_(III-N)/t_(S) of thickness t_(III-N) of the group III nitridefilm to thickness t_(S) of support substrate 11 was 0.02 or more and 1or less.

Reference Example II-G

Referring to FIG. 12, group III nitride composite substrate 1 wasfabricated in a similar manner to Reference Example II-A, except that:an Al₂O₃—SiO₂ composite oxide substrate (with respect to the wholesubstrate, Al₂O₃ was 88% by mass and SiO₂ was 12% by mass) was used assupport substrate 11; support substrate 11 was subjected to grinding andfinish polishing under the condition that action coefficient FE was5.9×10⁻¹⁷ m²/s to thereby have a thickness of 250 μm; group III nitridecomposite substrates 1 were fabricated with different diameters in arange of 75 mm to 150 mm; and obtained group III nitride compositesubstrate 1 was further cleaned. Here, support substrate 11 had athermal conductivity of 15 W·m⁻¹·K⁻¹ and a Young's modulus of 270 GPa.Regarding the cleaning method, a combination of scrub cleaning with asurfactant and pure water, two-fluid cleaning with hydrochloric acid orTMAH (tetramethylammonium hydroxide) and pure water, and megasoniccleaning with hydrochloric acid or TMAH and pure water was used.

2. Fabrication of Group III Nitride Semiconductor Device

Referring to FIG. 11, in the present reference example, an SBD wasfabricated as group III nitride semiconductor device 4. On main surface13 m of group III nitride film 13 of group III nitride compositesubstrate 1 shown in FIG. 6, the MOVPE method was used to epitaxiallygrow n⁺-GaN layer 28 (having a carrier concentration of 2×10¹⁸ cm⁻³)with a thickness of 2 μm and n⁻-GaN layer 29 (having a carrierconcentration of 5×10¹⁵ cm⁻³) with a thickness of 7 μm in this order toform group III nitride layer 20, and accordingly a laminated group IIInitride composite substrate was obtained.

Next, on n⁻-GaN layer 29, an Ni layer having a thickness of 4 nm and anAu layer having a thickness of 200 nm were successively formed by the EB(Electron Beam) deposition method, and annealed into an alloy, tothereby form first electrode 30 serving as a Schottky electrode. Thediameter of the electrode was set to 200 μm. On first electrode 30, a Tilayer having a thickness of 200 nm, a Pt layer having a thickness of 100nm, and an Au layer having a thickness of 1000 nm were successivelyformed by the EB deposition method to thereby form a pad electrode (notshown).

From the laminated group III nitride composite substrate, the supportsubstrate and the joint film were removed by grinding. For the grinding,a vitrified grinding wheel containing diamond abrasive grains having anaverage grain size of 40 μm to 50 μm was used.

Referring to FIG. 11, on main surface 13 n of group III nitride film 13having been exposed by removal of the support substrate and the jointfilm from the laminated group III nitride substrate, a Ti layer having athickness of 20 nm, an Al layer having a thickness of 200 nm, and an Aulayer having a thickness of 300 nm were successively formed by the EBdeposition method, and then annealed to form second electrode 50 servingas an ohmic electrode. In this way, group III nitride semiconductordevice 4 serving as an SBD was obtained.

In a similar manner to Reference Example II-A, for group III nitridecomposite substrate 1, mean value ms of the RMS of support-substrate11-side main surface 11 n was calculated and summarized in Table 18.Further, for group III nitride composite substrate 1, the concentrationof impurity metal atoms in group III-nitride-film 13-side main surface13 m was measured by the TXRF (total reflection x-ray fluorescence)method and summarized in Table 18. Here, the measurements by the TXRFmethod were done by means of a tungsten (W) x-ray source under acondition that the angle of incidence was 0.05°.

In the following way, the yield of group III nitride semiconductordevices 4 was also calculated. Specifically, for SBDs which were groupIII nitride semiconductor devices 4, current-voltage characteristics inthe reverse direction were measured. The SBDs were classified intonon-defective devices meeting a standard that the breakdown voltage ofthe SBD was 300 V or more and defective devices failing to meet thisstandard, and the percentage of the ratio determined by dividing thenon-defective devices by the sum of the non-defective devices and thedefective devices was defined as a yield. The yield of the group IIInitride semiconductor devices is summarized in Table 18.

TABLE 18 Reference Example II-G II-G1 II-G2 II-G3 II-G4 II-G5 II-G6II-G7 II-G8 diameter D (mm) 75 75 75 75 75 100 100 150 concentration ofimpurity 10 100 300 1000 2000 8 100 10 metal atoms (×10¹⁰ atoms/cm²)m_(s) (nm) 3.2 3.2 3.2 3.2 3.2 3.2 3.2 3.2 ratio s_(s)/m_(s) 0.25 0.250.25 0.25 0.25 0.25 0.25 0.25 device yield (%) 70 68 65 63 58 69 65 69

Referring to Table 18, group III nitride semiconductor devicesfabricated with a high yield were those using a group III nitridecomposite substrate with a diameter of 75 mm (this value meets 75 mm ormore) including a group III nitride film with a thickness of 110 μm,where the concentration of impurity metal atoms in the groupIII-nitride-film-side main surface was 3×10¹² atoms/cm² or less.

Reference Example II-H

Referring to FIGS. 14 and 15, group III nitride composite substrate 1and group III nitride semiconductor device 4 were fabricated in asimilar manner to Reference Example II-A except that: a substrate havinga diameter of 75 mm and a thermal conductivity between 2 W·m⁻¹·K⁻¹ and300 W·m⁻¹·K⁻¹ was used as support substrate 11; grinding and polishingwere performed from main surface 13 m opposite to the bonded mainsurface of group III nitride film donor substrate 13D with a diameter of75 mm to thereby allow group III nitride film 13 to have a thickness of110 μm; and support-substrate 11-side main surface 11 n wasfinish-polished under the condition that action coefficient FE was9.0×10⁻⁷ m²/s. Here, for grinding of group III nitride film donorsubstrate 13D, vitrified abrasive grains including diamond abrasivegrains having an average grain size of 25 μm to 35 μm were used. Thethermal conductivity of support substrate 11 was obtained by adjustingthe oxide material content and the sintering conditions.

In a similar manner to Reference Example II-A, the yield of group IIInitride semiconductor devices 4 was calculated and summarized in Table19.

TABLE 19 Reference Example II-H II-H1 II-H2 II-H3 II-H4 II-H5 II-H6II-H7 II-H8 II-H9 thermal conductivity of 2 3 5 10 30 120 210 280 300support substrate λ_(S) (W · m⁻¹ · K⁻¹) m_(s) (nm) 2.5 2.5 2.5 2.5 2.52.5 2.5 2.5 2.5 ratio s_(s)/m_(s) 0.08 0.08 0.08 0.08 0.08 0.08 0.080.08 0.08 device yield (%) 55 63 65 67 67 66 64 61 55

Referring to Table 19, group III nitride semiconductor devicesfabricated with a high yield were those using a group III nitridecomposite substrate with a diameter of 75 mm (this value meets 75 mm ormore) including a group III nitride film with a thickness of 110 μm(this value meets 10 μm or more and 250 μm or less) and including asupport substrate having a thermal conductivity of 3 W·m⁻¹·K⁻¹ or moreand 280 W·m⁻¹·K⁻¹ or less.

Reference Example III-A to Reference Example III-D according toReference Invention III will be illustrated below.

(1) Fabrication of Group III Nitride Composite Substrate ReferenceExample III-A

In the following, with reference to FIG. 12, a group III nitridecomposite substrate according to the reference example will bedescribed.

First, as shown in FIG. 12 (A), an Al₂O₃—SiO₂ composite oxide substrate(with respect to the whole substrate, Al₂O₃ was 85% by mass and SiO₂ was15% by mass) having a diameter of 75 mm and a thickness of 300 μm wasprepared for use as support substrate 11. Support substrate 11 had athermal conductivity of 10 W·m⁻¹·K⁻¹ and a Young's modulus of 250 GPa.

Subsequently, main surfaces 11 m, 11 n of the two opposite sidesrespectively of support substrate 11 were subjected to rough polishingwith a copper-based surface plate, intermediate polishing with atin-based surface plate, and finish polishing with a nonwoven polishingpad, for which a diamond slurry was used as an abrasive. The finishpolishing was performed under the condition that action coefficient FEwas 4×10⁻¹⁷ m²/s or more and 1×10⁻¹⁶ m²/s or less.

Next, on finish-polished main surface 11 m of support substrate 11, anSiO₂ film of 800 nm in thickness was grown by the PE-CVD (PlasmaEnhanced-Chemical Vapor Deposition) method, and annealed in a nitrogenatmosphere at 800° C. for an hour.

Next, CMP using a slurry with a pH of 10 containing colloidal silicaabrasive grains having an average grain size of 40 nm was performed tothereby form joint film 12 a of 400 nm in thickness having its mainsurface 12 am mirror-finished so that the RMS (root mean squareroughness) was 0.3 nm or less. Subsequently, in order to remove thecolloidal silica abrasive grains used for the CMP, non-abrasivepolishing cleaning with a KOH aqueous solution, polishing cleaning withpure water, and megasonic cleaning (cleaning with ultrasonic waves in amegasonic frequency range of 500 kHz to 5 MHz) with pure water wereperformed.

As shown in FIG. 12 (B), a GaN crystal body having a diameter of 75 mmand a thickness of 8 mm was prepared for use as group III nitride filmdonor substrate 13D. Subsequently, a surface-to-be-bonded of group IIInitride film donor substrate 13D was subjected to mechanical polishingand CMP so that it was planarized to have an RMS of 2 nm or less. AnSiO₂ film of 800 nm in thickness was grown thereon by the PE-CVD method,and annealing was performed in a nitrogen atmosphere at 800° C. for anhour. Subsequently, CMP using a slurry with a pH of 10 containingcolloidal silica abrasive grains having an average grain size of 40 nmwas performed to thereby form joint film 12 b of 500 nm in thicknesshaving its main surface 12 bn mirror-finished so that the RMS was 0.3 nmor less. Subsequently, in order to remove the colloidal silica abrasivegrains used for the CMP, non-abrasive polishing cleaning with a KOHaqueous solution, polishing cleaning with pure water, and megasoniccleaning with pure water were performed.

Here, group III nitride film donor substrate 13D was produced throughgrowth by the HVPE method using a GaAs substrate as a base substrate.Group III nitride film donor substrate 13D had its electricalconductivity of n-type, its dislocation density of 1×10⁸ cm⁻², and itscarrier concentration of 1×10¹⁷ cm⁻³.

Next, as shown in FIG. 12 (C), main surface 12 am of joint film 12 a andmain surface 12 bn of joint film 12 b were bonded together to therebyproduce joined substrate 1L in which support substrate 11 and group IIInitride film 13 were bonded to each other with joint film 12 interposedtherebetween. After they were bonded together, joined substrate 1L wasannealed by being heated in a nitrogen gas atmosphere to 800° C., tothereby increase the joint strength.

Next, as shown in FIG. 12 (D), group III nitride film donor substrate13D in joined substrate 1L was cut, with a wire saw, along a planelocated inwardly at a depth of 40 μm from the bonded surface where groupIII nitride film donor substrate 13D was bonded to joint film 12, tothereby produce group III nitride composite substrate 1 in which supportsubstrate 11 and group III nitride film 13 which was a GaN thin filmwere bonded together with joint film 12 interposed therebetween.

As the wire, a fixed-abrasive wire with a diameter of 180 μm on whichdiamond abrasive grains were electrodeposited was used. As for thecutting method, in order to reduce the cut resistance and enhance thethickness precision and the flatness of the cut plane, the method wasused that caused the wire to swing and caused group III nitride filmdonor substrate 13D to vibrate in synchronization therewith. Theresistance coefficient for cutting with the wire saw was set to 4200 N.

After cutting, group III nitride film 13 of group III nitride compositesubstrate 1 was subjected to mechanical polishing and CMP. At this time,a diamond slurry was used as an abrasive, and rough polishing with acopper-based surface plate and intermediate polishing with a tin-basedsurface plate were performed. Further, a colloidal silica slurry with pH11 (a slurry of pH 11 containing colloidal silica abrasive grains havingan average grain size of 60 nm) was used to perform finish polishingwith a nonwoven polishing pad. In order to make the thickness of groupIII nitride film 13 uniform, the composite substrate was mounted on theapparatus for the CMP, by a method according to which the shape of thesubstrate was corrected in advance by vacuum chuck suction, andthereafter the composite substrate was suction-fixed onto the apparatus.The finish polishing was performed under the condition that actioncoefficient FE was 7×10⁻¹⁴ m²/s. The thickness of finish-polished groupIII nitride film 13 was 110 μm.

[(2) Fabrication of Group III Nitride Semiconductor Device]

In the following, with reference to FIG. 16, an SBD (Schottky BarrierDiode) which is a group III nitride semiconductor device according tothe reference example will be described.

First, as shown in FIG. 16 (A), on group III-nitride-film 13-side mainsurface 13 m of group III nitride composite substrate 1, the MOVPEmethod was used to epitaxially grow n⁺-GaN layer 28 (having a carrierconcentration of 2×10¹⁸ cm⁻³) with a thickness of 2 μm and n⁻-GaN layer29 (having a carrier concentration of 5×10¹⁵ cm⁻³) with a thickness of 7μm in this order to form group III nitride layer 20, and accordinglylaminated group III nitride composite substrate 2 was obtained.

Next, as shown in FIG. 16 (B), on n⁻-GaN layer 29 which was the topmostlayer of group III nitride layer 20 of laminated group III nitridecomposite substrate 2, an Ni layer having a thickness of 4 nm and an Aulayer having a thickness of 200 nm were successively formed by theelectron beam deposition method (hereinafter also referred to as EB(Electron Beam) deposition method), and annealed into an alloy, tothereby form first electrode 30 serving as a Schottky electrode. At thistime, the diameter of first electrode 30 was set to 200 μm. Further, onfirst electrode 30, a Ti layer having a thickness of 200 nm, a Pt layerhaving a thickness of 100 nm, and an Au layer having a thickness of 1000nm were successively formed by the EB deposition method to thereby forma pad electrode 33.

As device support substrate 40, an Mo substrate was prepared. On devicesupport substrate 40, a Ti layer having a thickness of 200 nm, a Ptlayer having a thickness of 100 nm, and an Au layer having a thicknessof 1000 nm were successively formed by the EB deposition method tothereby form pad electrode 43. On pad electrode 43, an AuSn solder filmwas formed as joint metal film 44.

Next, joint metal film 44 was bonded to pad electrode 33 to therebyproduce laminated substrate 3.

Next, as shown in FIG. 16 (C), from laminated substrate 3, supportsubstrate 11 and joint film 12 in group ITT nitride composite substrate1 were etched away by means of hydrofluoric acid.

Next, as shown in FIG. 16 (D), on group III nitride film 13 having beenexposed by removal of support substrate 11 and joint film 12 fromlaminated substrate 3, a Ti layer having a thickness of 20 nm, an Allayer having a thickness of 200 nm, and an Au layer having a thicknessof 300 nm were successively formed by the EB deposition method, and thenannealed to form second electrode 50 which was an ohmic electrode. Ondevice support substrate 40, a Ti layer having a thickness of 20 nm andan Au layer having a thickness of 300 nm were successively formed by theEB deposition method, and annealed to thereby form device supportsubstrate electrode 45. In this way, group III nitride semiconductordevice 4 which was an SBD was obtained.

The yield of group III nitride semiconductor devices 4 thus obtained wascalculated in the following way. Specifically, for SBDs, current-voltagecharacteristics in the reverse direction were measured. The SBDs wereclassified into non-defective devices meeting a standard that thebreakdown voltage was 250 V or more and defective devices failing tomeet this standard, and the percentage of the ratio determined bydividing the non-defective devices by the sum of the non-defectivedevices and the defective devices was defined as a yield.

In accordance with the method as described above, group III nitridecomposite substrates each including the joint film having a thicknessvariation shown in Table 20, and group III nitride semiconductor devicesfor which the composite substrates were used were fabricated.

These group III nitride composite substrates were each a compositesubstrate having a diameter of 75 mm (namely 75 mm or more) in which asupport substrate and a group III nitride film having a thickness of 110μm (namely 10 μm or more and 250 μm or less) were bonded to each other.

A relation between the thickness variation of the joint film and theyield of the group III nitride semiconductor devices calculated by theabove-described method is shown in Table 20.

TABLE 20 Reference Example III-A III-A1 III-A2 III-A3 III-A4 III-A5III-A6 III-A7 thickness variation 1 2 5 13 25 40 47 of joint film (%)device yield (%) 35 69 77 84 76 71 33

As clearly seen from Table 20, the yield of the semiconductor devices(III-A2 to III-A6) in which group III nitride composite substrates eachhaving a thickness variation of the joint film of 2% or more and 40% orless were used was higher than that of the semiconductor devices (III-A1and III-A7) in which group III nitride composite substrates failing tosatisfy this condition were used.

Reference Example III-B

In a similar manner to Reference Example III-A, group III nitridecomposite substrates each including a support substrate and a group IIInitride film joined to each other with the shear joint strength and theratio of the joint area shown in Table 21, and group III nitridesemiconductor devices for which these composite substrates were usedwere fabricated.

These group III nitride composite substrates were each a compositesubstrate having a diameter of 75 mm (namely 75 mm or more) in which asupport substrate and a group III nitride film having a thickness of 110μm (namely 10 μm or more and 250 μm or less) were bonded to each other.

A relation between the shear joint strength and the ratio of the jointarea and the yield of the group III nitride semiconductor devicescalculated by the above-described method is shown in Table 21.

TABLE 21 Reference Example III-B III-B1 III-B2 III-B3 III-B4 III-B5III-B6 III-B7 III-B8 III-B9 shear joint strength (MPa) 2 4 4 10 30 33 4040 60 ratio of joint area (%) 80 60 99 70 90 51 60 99 99 device yield(%) 25 66 69 75 84 39 64 74 56

As clearly seen from Table 21, the yield of the semiconductor devices(III-B2 to III-B5, III-B7, and m-B8) in which used group III nitridecomposite substrates each having a shear joint strength between thesupport substrate and the group III nitride film of 4 MPa or more and 40MPa or less and a ratio of the joint area between the support substrateand the group III nitride film of 60% or more and 99% or less was higherthan that of the semiconductor devices (III-B1, III-B6, and III-B9) inwhich group III nitride composite substrates failing to satisfy thiscondition were used.

Reference Example III-C

In a similar manner to Reference Example III-A except for the followingconditions (i) to (v), group III nitride composite substrates accordingto Reference Example III-C and group III nitride semiconductor devicesfor which these composite substrates were used were fabricated:

(i) a composite oxide substrate selected from an Al₂O₃—SiO₂ compositeoxide substrate (composite oxide substrate in which, with respect to thewhole substrate, Al₂O₃ was 82% by mass and SiO₂ was 18% by mass), amullite-YSZ substrate, and a mullite substrate was used as supportsubstrate 11;

(ii) different diameters in a range of 75 mm to 150 mm were used;

(iii) joint film 12 was grown by the AP-CVD (AtmosphericPressure-Chemical Vapor Deposition) method;

(iv) support-substrate 11-side main surface 11 n was finish-polishedunder the condition that action coefficient FE was 8.5×10⁻¹⁷ m²/s ormore and 1×10⁻¹⁶ m²/s or less; and

(v) finish-polished group III nitride films 13 having differentthicknesses in a range of 10 μm to 250 μm were used.

It should be noted that the thickness variations of the joint films ingroup III nitride composite substrates in Reference Example III-C wereall 5% (namely 2% or more and 40% or less).

The relation between the characteristics of the group III nitridecomposite substrates in Reference Example III-C and the yield of groupIII nitride semiconductor devices for which the composite substrates areused is shown in Table 22.

TABLE 22 Reference Example III-C III-C1 III-C2 III-C3 III-C4 III-C5III-C6 III-C7 III-C8 III-C9 III-C10 III-C11 III-C12 III-C13 diameter(mm) 75 75 75 75 100 100 100 100 100 100 100 150 150 type of supportAl₂O₃—SiO₂ mullite-YSZ Al₂O₃—SiO₂ mullite Al₂O₃—SiO₂ substrateα_(III-N)/α_(S)(—) 1 1 1 1 1 1 1 1.25 0.9 0.75 1.2 1.2 1t_(III-N)/t_(S)(—) 0.02 0.1 0.3 0.6 1 0.4 0.9 1 1 1 0.4 1 0.2 thickness5 5 5 5 5 5 5 5 5 5 5 5 5 variation of joint film (%) device yield (%)62 70 72 69 69 76 65 68 75 67 60 55 69

In Table 22, α_(III-N)/α_(S) represents the ratio of thermal expansioncoefficient α_(III-N) of the group III nitride film to thermal expansioncoefficient α_(S) of the support substrate, and t_(III-N)/t_(S)represents the ratio of thickness t_(III-N) of the group III nitridefilm to thickness t_(S) of the support substrate.

As clearly seen from Table 22, among the group III nitride compositesubstrates having a diameter of 75 mm or more and including the supportsubstrate and the group III nitride film with a thickness of 10 μm ormore and 250 μm or less that were bonded to each other, where the jointfilm had a thickness variation of 5% (namely 2% or more and 40% orless), group III nitride composite substrates having t_(III-N)/t_(S) of0.02 or more and 1 or less could be used to manufacture group IIInitride semiconductor devices with a particularly high yield.

In addition to the above, in the composite substrates havingα_(III-N)/α_(S) of 0.75 or more and 1.25 or less, crack did not occur atall, and their yield was also high.

Reference Example III-D

In a similar manner to Reference Example III-A except that a supportsubstrate having thermal conductivity λ_(S) shown in Table 23 was used,group III nitride composite substrates and group III nitridesemiconductor devices for which these composite substrates were usedwere fabricated.

The relation between thermal conductivity λ_(S) of the support substrateand the yield of group III nitride semiconductor devices is shown inTable 23.

TABLE 23 Reference Example III-D III-D1 III-D2 III-D3 III-D4 III-D5III-D6 III-D7 III-D8 III-D9 thermal conductivity of 2 3 5 10 30 120 210280 300 support substrate λ_(S) (W · m⁻¹ · K⁻¹) thickness variation of 55 5 5 5 5 5 5 5 joint film (%) device yield (%) 55 63 65 67 67 66 64 6155

As clearly seen from Table 23, among the group III nitride compositesubstrates having a diameter of 75 mm (namely 75 mm or more) andincluding the support substrate and the group III nitride film with athickness of 110 μm (namely 10 μm or more and 250 μm or less) that werebonded to each other, where the joint film had a thickness variation of5% (namely 2% or more and 40% or less), group III nitride compositesubstrates having a thermal conductivity λ_(S) of 3 W·m⁻¹·K⁻¹ or moreand 280 W·m⁻¹·K⁻¹ or less could be used to manufacture group III nitridesemiconductor devices with a particularly high yield.

It should be construed that the embodiments and examples disclosedherein are given by way of illustration in all respects, not by way oflimitation. It is intended that the scope of the present invention isdefined by claims, not by the description above, and encompasses allmodifications and variations equivalent in meaning and scope to theclaims.

REFERENCE SIGNS LIST

1 group III nitride composite substrate; 1L, 1LS joined substrate; 2laminated group III nitride composite substrate; 3 laminated substrate;4 group III nitride semiconductor device; 5D, 5Dr support-incorporatedgroup III nitride film donor substrate; 11 support substrate; 11 m, 11n, 12 am, 12 bn, 13 m, 13 n main surface; 12, 12 a, 12 b, 14 joint film;13 group III nitride film; 13D, 13Dr group III nitride film donorsubstrate; 13 h main-surface through hole; 13 i ion implantation region;15 group III nitride film donor substrate support; 20 group III nitridelayer; 21 n-GaN layer; 22 n-In_(0.05)Ga_(0.95)N layer; 23 active layer;24 p-Al_(0.09)Ga_(0.91)N layer; 25 p-GaN layer; 26 GaN layer; 27Al_(0.2)Ga_(0.8)N layer; 28 n⁺-GaN layer; 29 n⁻-GaN layer; 30 firstelectrode; 33, 43 pad electrode; 40 device support substrate; 44 jointmetal film; 45 device support substrate electrode; 50 second electrode;60 source electrode; 70 drain electrode; 80 gate electrode; 100 jointinterface; 100 b joined region; 100 n non-joined region; 111 n, 112 n,121 n, 122 n non-joined partial region; 201 first-conductivity-type GaNlayer; 202 first-conductivity-type Al_(s)Ga_(1-s)N layer; 203 lightemission layer; 204 second-conductivity-type Al_(t)Ga_(1-t)N layer; 205second-conductivity-type GaN layer

1-10. (canceled) 11: A group III nitride composite substrate with adiameter of 75 mm or more including a support substrate and a group IIInitride film having a thickness of 10 μm or more and 250 μm or less thatare bonded to each other, the group III nitride composite substratecomprising a joint film interposed between the support substrate and thegroup III nitride film and joining the support substrate and the groupIII nitride film to each other, the joint film having a thicknessvariation of 2% or more and 40% or less. 12: A group III nitridecomposite substrate with a diameter of 75 mm or more including a supportsubstrate and a group III nitride film having a thickness of 10 μm ormore and 250 μm or less that are bonded to each other, the group IIInitride composite substrate comprising a joint film interposed betweenthe support substrate and the group III nitride film and joining thesupport substrate and the group III nitride film to each other, a shearjoint strength between the support substrate and the group III nitridefilm being 4 MPa or more and 40 MPa or less, a ratio of a joint areabetween the support substrate and the group III nitride film being 60%or more and 99% or less. 13: The group III nitride composite substrateaccording to claim 11, wherein a ratio α_(III-N)/α_(S) of a thermalexpansion coefficient α_(III-N) of the group III nitride film to athermal expansion coefficient αS of the support substrate is 0.75 ormore and 1.25 or less, and a ratio t_(III-N)/t_(S) of a thickness tin-Nof the group III nitride film to a thickness t_(S) of the supportsubstrate is 0.02 or more and 1 or less. 14: The group III nitridecomposite substrate according to claim 11, wherein the support substratehas a thermal conductivity λ_(S) of 3 W·m⁻¹·K⁻¹ or more and 280W·m⁻¹·K⁻¹ or less. 15: The group III nitride composite substrateaccording to claim 11, wherein the support substrate has a Young'smodulus E_(S) of 150 GPa or more and 500 GPa or less. 16: The group IIInitride composite substrate according to claim 11, wherein the diameterof the group III nitride composite substrate is 125 mm or more and 300mm or less. 17: A laminated group III nitride composite substratecomprising: the group III nitride composite substrate as recited inclaim 11; and at least one group III nitride layer disposed on a groupIII-nitride-film-side main surface of the group III nitride compositesubstrate. 18: A group III nitride semiconductor device comprising: thegroup III nitride film included in the group III nitride compositesubstrate as recited in claim 11; and at least one group III nitridelayer disposed on the group III nitride film. 19: A method formanufacturing the group III nitride composite substrate as recited inclaim 11, the method comprising: forming a joined substrate with adiameter of 75 mm or more by bonding a support substrate and a group IIInitride film donor substrate to each other; and forming the group IIInitride composite substrate by cutting the group III nitride film donorsubstrate in the joined substrate along a plane located inwardly at apredetermined distance from a bonded main surface of the group IIInitride film donor substrate. 20: A method for manufacturing the groupIII nitride composite substrate as recited in claim 11, the methodcomprising: forming a joined substrate with a diameter of 75 mm or moreby bonding a support substrate and a group III nitride film donorsubstrate to each other; and forming the group III nitride compositesubstrate by performing at least one of grinding, polishing, and etchingon a main surface of the group III nitride film donor substrate in thejoined substrate, the main surface being opposite to a bonded mainsurface of the group III nitride film donor substrate. 21: A method formanufacturing a group III nitride semiconductor device, the methodcomprising: preparing the group III nitride composite substrate asrecited in claim 11; and growing at least one group III nitride layer onthe group III nitride film of the group III nitride composite substrate.22: The method for manufacturing a group III nitride semiconductordevice according to claim 21, further comprising: bonding a devicesupport substrate onto the group III nitride layer; and removing thesupport substrate from the group III nitride composite substrate.